TECHNIQUE OF See also:

• PHOTOGRAPHY (Gr. bias, light, and ypa w, to write)
• PHOTOGRAPHY, CELESTIAL

30 „ 4. See also:

• SILVER

4 is dissolved in z oz. of water, and No. 3 is allowed to swell up in i oz. of water, and is then dissolved by See also:

• HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

The gelatin is next drained by straining canvas over a jar and turning out the threads on to it, after which it is placed in a flask, and warmed till it dissolves; half an See also:

• OUNCE

When set the emulsion is washed, drained, and redissolved as before; but in See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

If the alkaline developer be used, the following may be taken as a good See also:

• STANDARD
• STANDARD, BATTLE OF THE

After thorough washing the negative is treated with ferrous oxalate. This process can be repeated till sufficient density is attained. With most other methods with mercury the image is See also:

• APT

The albumen is allowed two or three days to See also:

• SETTLE
• SETTLE, ELKANAH (1648–1724)

This gilded the darkened parts of the print which light had reduced to the semi-metallic state: and on the removal of the chloride by means of hyposulphite an image composed of metallic silver, an organic salt of silver and gold was See also:

• LEFT

The following may be taken as two typical toning-See also:

• BATHS

This was an important See also:

• FACTOR (from Lat. facere, to make or do)

If this be the case, it seems probable that the destruction of the hyposulphite by time is not the occasion of fading, but that its hygroscopic See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

2 are next mixed with the collodion, and finally 30 parts of No. 3. This forms an emulsion of silver chloride and also contains citric acid and silver nitrate. The defect of this emulsion is that it contains a large proportion of soluble salts, which are apt to crystallize out on drying, more particularly if it be applied to glass plates. The addition of the citric acid and the excess of silver nitrate is the See also:

• KEY
• KEY (in O. Eng. tang; the ultimate origin of the word is unknown; it appears only in Old Frisian kei of other Teutonic languages; until the end of the 17th century the pronunciation was kay, as in other words in O. Eng. ending in aeg; cf. daeg, day; clang
• KEY (the phraseograms being indicated by hyphens)
• KEY, SIR ASTLEY COOPER (1821-1888)
• KEY, THOMAS HEWITT (1799-1875)

1. Potassium citrate 40 Water 500 2 Silver nitrate 150 Water 500 Gelatin 300 Water 1700 Nos. 2 and 3 are mixed together whilst warm, and No. t is then gently added, the gelatin solution being kept in brisk agitation. This produces the emulsion of citrate and chloride of silver. The gelatin containing the suspended salts is heated for five minutes at boiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to paper or glass. The prints are of a beautiful colour, and seem to be fairly permanent. They may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed with the hyposulphite solution of the strength before given. Most, if not all, of the gelatin papers now extant are made somewhat after this manner. Printing with Salts of See also:

• URANIUM [symbol U, atomic weight 238.5 (0=16)]

One of the See also:

• ORIGINAL

See also:

• JOSEPH
• JOSEPH, COMTE DE (1785-1870)
• JOSEPH, FATHER (FRANCOIS LECLERC DU TREMBLAY) (1577-1638)

(2) that in printing the paper should have its unprepared See also:

• SIDE
• SIDE (mod. Eski Adalia)

See also:

• JOHNSON, ANDREW
• JOHNSON, ANDREW (1808–1875)
• JOHNSON, BENJAMIN (c. 1665-1742)
• JOHNSON, EASTMAN (1824–1906)
• JOHNSON, REVERDY (1796–1876)
• JOHNSON, RICHARD (1573–1659 ?)
• JOHNSON, RICHARD MENTOR (1781–1850)
• JOHNSON, SAMUEL (1709-1784)
• JOHNSON, SIR THOMAS (1664-1729)
• JOHNSON, SIR WILLIAM (1715–1774)
• JOHNSON, THOMAS

The gelatin, sugar and soap are put in water and allowed to stand for an hour, and then melted, the liquid afterwards receiving the 3. colours, which have been ground on a slab. The mixture is filtered I the whole became coagulated rendered these unmanageable. It through fine muslin. In making the tissue in large quantities the at last occurred to him that if the hardening action of light were two ends of a piece of See also:

• ROLL (O. Fr. rolle, roulle, mod. role, Lat. rotulus, dim. of rota, wheel)
• ROLL, ALFRED PHILIPPE (1846– )

This is effected by turning up about t in. from the by See also:

• OHM, GEORG SIMON (1787-1854)

Sup- the application of the principle might lead to important results. posing it is judged that a sheet of tissue under some one negative It was just these results which Woodbury obtained, and for which ought to be exposed to light corresponding to a given number of he was entitled to the fullest See also:

• CREDIT (Lat. credere, to believe)

Following image (reversed as regards right and left) on the flexible support. very closely upon Asser, J. W. See also:

• OSBORNE

which is a point of practical value, since single-transfer are better than double-transfer prints. I Printing with Salts of See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

Albert of Munich. He had been experimenting for many years, endeavouring to make the gelatin films more durable than those of Tessie de Motay. He added gum-resins, alum, See also:

• TANNIN, or TANNIC ACID

Becquerel preferred to chlorinize the plate by See also:

• IMMERSION (Lat. immersio, dipping),

In the Bulletin de la Societe FranQaise (1874) Colonel St Florent described experiments which he made with the same See also:

• OBJECT

In reference to these coloured images on paper it must not be forgotten that pure salts of silver are not being dealt with as a See also:

• RULE

Lumiere used gelatin plates dyed withappropriate dyes (orthochromatic plates); the exposure was much diminished, and very excellent representations were produced of all natural colours. The See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

Hereupon I poured some See also:

• CAUSTIC (Gr. rcavvraubs, burning)

The mercury vapour volatilizes to a certain extent and fills the tube. When exposed to light chlorine is liberated from the chloride, and See also:

• CALOMEL

One is based on the supposition that the molecules of the salt can rearrange their atoms under the vibrations caused by the See also:

• ETHER, (C2H5)2O

We have already described Seebeck's (181o) experiments on silver chloride with the spectrum whereby he obtained coloured photographs, but Scheele in 1777 allowed a spectrum to fall on the same material, and found that it blackened much more readily in the violet rays than in any other. See also:

• SENEBIER, JEAN (1742–1809)

Effect of Dyes on Sensitive Films.—In 1874 Dr H. W. Vogel of See also:

• BERLIN
• BERLIN, CONGRESS AND TREATY OF
• BERLIN, ISAIAH (1725–1799)

Correct Rendering of Colours in Monochrome.—In Plate IV., fig. 14 the sensitiveness of a plate stained with homocol is shown, and it is evident that as it is sensitive throughout the visible spectrum there must be some means of cutting off by a transparent screen so much of the spectrum luminosity at different parts that every colour having the same lurnim*ity to the eye shall be shown on a negative of equal density. When this is done the relative luminosities of all colours will be shown by the same relative densities ( 1e . ) blue and an orange glass can be very accurately D. measured; if I-in. squares of these coloured glasses, (I.e.) together with a white glass of the same See also:

• AREA

AgI prepared in bath, treated with BI, washed, redipped in silver bath, developed with pyrogallic acid. Grey AgBr in gelatin, developed alkaline or ferrous oxalate . . . Orange AgBr in collodion or gelatin, alkaline ferrous oxalate or acid developer. Green AgBr in collodion, developed ferrous oxalate . . . AgCl in•collodion, excess of AgNOa or NaCl present, ferrous citrate or acid development. AgI+AgBr, washed from AgNO3 D. or in a print by different depths of greys. Abney (Ie.) devised a sensitometer which should be used to D.e)'ascertain the colour of the screen that should be employed. By proper means the luminosity of D. the light of day coming through a red, a green, a 3AgI+ AgBr + AgNOx collodion, wet plate. acid or alkaline developer [?.=print; D. = developed; I.e. = long exposure]. two for their See also:

• HISTORICAL

Herschel (Phil. Trans., 1840). He describes many experiments, which 'See Abney, "Destruction of the Photographic Image," Phil. Mae. (1878), vol. v.; also Proc. Roy. Soc. (1878), vol. xxvil. blue glass, for instance, would not be covered by the disk at all, while opposite the white square the disk would have an aperture of an angle of r8°. When a plate is exposed behind the row of glass squares, with the light passing through the rotating disk, having the appropriate apertures for each glass, the negative obtained would under ordinary conditions, show square patches of very different opacity. A light-filter of some transparent colour, if placed in the path of the light, will alter the opacities, and eventually one can be found which will only allow such coloured light to be transmitted as will cause all the opacities in the negative to be the same. As the luminosities of the white light passing through the glasses are made equal, and as the photographic deposits are also rendered equal, this light-filter, if used in front of the camera lens, will render all coloured objects in correct monochrome luminosity.

Another plan, based on the same principles, is to place segments of annuluses of See also:

• VERMILION

2 of a shorter one. It should be noticed that the solution of potassium bichromate absorbs those rays alone which are effective in altering the bichromate. This change is only possible in the presence of organic matter of some kind, such as gelatin or albumen. Action of the Spectrum on Asphaltum.—This seems to be continued into and below the red, the blue rays, however, are the most effective. The action of light on this body is to render it less soluble in its usual solvents. Action of the Spectrum on Salts of Iron.—The commonest ferric salt in use is the oxalate, by which the beautiful platinotype prints are produced. We give this as a See also:

• REPRESENTATION

This is one of the few instances in which these light-waves of See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

J. H. Schulze . . 1727 Nitrate solution mixed with chalk, gives in See also:

• SUNSHINE

See also:

• FISCHER
• FISCHER, EMIL (1852– )
• FISCHER, ERNST KUNO BERTHOLD (1824–1907)

. . . 1814 Iodide by action of iodine (on See also:

• DAGUERRE, LOUIS JACQUES MANDE (1789-1851)

. 184o and iodine (on metallic silver). See also:

• BALARD, ANTOINE JEROME (1802-1876)

. 1800 Oxalate Bergmann . . 1779 Benzoate Trommsdorf . 1793 Citrate Vauquelin 1798 Kinate See also:

• HENRY
• HENRY (1129-1195)
• HENRY (c. 1108-1139)
• HENRY (c. 1174–1216)
• HENRY (Fr. Henri; Span. Enrique; Ger. Heinrich; Mid. H. Ger. Heinrich and Heimrich; O.H.G. Haimi- or Heimirih, i.e. " prince, or chief of the house," from O.H.G. heim, the Eng. home, and rih, Goth. reiks; compare Lat. rex " king "—" rich," therefore " mig
• HENRY, EDWARD LAMSON (1841– )
• HENRY, JAMES (1798-1876)
• HENRY, JOSEPH (1797-1878)
• HENRY, MATTHEW (1662-1714)
• HENRY, PATRICK (1736–1799)
• HENRY, PRINCE OF BATTENBERG (1858-1896)
• HENRY, ROBERT (1718-1790)
• HENRY, VICTOR (1850– )
• HENRY, WILLIAM (1795-1836)

. . 182o Phosphide by vapour of phos- phorus (on metallic silver). Scheele. . . . 1777 Gold. Oxide Chloride on paper Hellot . . . . 1737 Chloride on silk Fulhame . .. 1794 Chloride in ethereal solution Rumford . . 1793 Chloride with ferrocyanide and Hunt . . .

1844 ferricyanide of potassium. See also:

• DOBEREINER, JOHANN WOLFGANG (178o--1849)

Platinum. Chloride in ether Gehlen . . . 1804 Chloride with lime Herschel . . . 184o Iodide Herschel . . . 184o Bromide Hunt 1844 Cyanide Dobereiner . . 1828 Double chloride of platinum and potassium. Gay-Lussac and 1811 Mercury. Oxide (mercurous) Oxide See also:

• THENARD, LOUIS JACQUES (1777-1857)

. , . Oxide (mercuric) Davy . . . 1797 Oxide (more accurate observa- , See also:

• ABILDGAARD, NIKOLAJ ABRAHAM (1744—1809)

1976 Sulphate See also:

• MEYER, CHRISTIAN ERICH HERMANN VON (18or-1869)
• MEYER, HEINRICH AUGUST WILHELM (1800-1873)
• MEYER, JULIUS LOTHAR (183o-1895)
• MEYER, KONRAD FERDINAND (1825-1898)
• MEYER, VICTOR (1848-1897)
• MEYER, [MARIE] PAUL HYACINTHE (184o- )

. Bromide (mercuric) . . . . Lowig . . . . 1828 Iodide (mercurous) Torosewicz . . 1836 Artus . 1836 Iodide (mercuric) See also:

• FIELD (a word common to many West German languages, cf. Ger. Feld, Dutch veld, possibly cognate with O.E. f olde, the earth, and ultimately with root of the Gr. irAaror, broad)
• FIELD, CYRUS WEST (1819-1892)
• FIELD, DAVID DUDLEY (18o5-1894)
• FIELD, EUGENE (1850-1895)
• FIELD, FREDERICK (18o1—1885)
• FIELD, HENRY MARTYN (1822-1907)
• FIELD, JOHN (1782—1837)
• FIELD, MARSHALL (183 1906)
• FIELD, NATHAN (1587—1633)
• FIELD, STEPHEN JOHNSON (1816-1899)
• FIELD, WILLIAM VENTRIS FIELD, BARON (1813-1907)

. Davy . . . . Nitrate Herschel . . . 184o Sulphide (mercuric) • See also:

• VITRUVIUS (MARCUS VITRUVIUS POLLIO)

. . . See also:

• KLAPROTH, HEINRICH JULIUS (1783-1835)
• KLAPROTH, MARTIN HEINRICH (1743-1817)

Grotthus . . . 1818 Prussian blue Scopoli . . . 1783 Ferric citrate with ammonium .. Herschel . . . 184o Ferric tartrate . Herschel . . . 184o Chromate . Hunt .

. . . 1844 Copper. Gehlen . , . 1804 Chloride (cupric dissolved in ether). . A. Vogel . . . 1813 Oxalate with sodium Chromate Hunt . . . . 1844 Chromate with ammonium Carbonate Iodide . A.

Vogel , . . 1859 Sulphate Chloride- (cuprous) Copper plates (iodized) Kratoch . 1841 Talbot . : : 1841 Manganese. . See also:

• BRANDENBURG

Hunt . , . . 1844 Chloride Lead. Davy . . . 1802 Oxide Iodide See also:

• SCHONBEIN, CHRISTIAN FRIEDRICH (1799-1868)

. . . 1844 Acetate See also:

• NICKEL
• NICKEL (symbol Ni, atomic weight 58.68 (0=16))

. Various Substances. See also:

• COBALT (symbol Co, atomic weight 59)

1874 See also:

• RHODIUM [symbol Rh; atomic weight 102.9 (0=16)]

Chlorine and See also:

• MARSH (O. F. mersc, for merisc, a place full of "meres" or pools; cf. Ger. Meer, sea, Lat. mare)
• MARSH, ADAM (ADAM DE MARISCO) (d. c. 1258)
• MARSH, GEORGE PERKINS (1801-1882)
• MARSH, HERBERT (1757–1839)
• MARSH, NARCISSUS (1638-1713)
• MARSH, OTHNIEL CHARLES (1831-1899)

See also:

• PETIT, SIR DINSHAW MANECKJI, BART

A. D Yellow wax bleached Eudoxia macrembolitissa (purple loth cent. dye). See also:

• COLE, SIR HENRY (1808–1882)
• COLE, THOMAS (1801–1848)
• COLE, TIMOTHY (1852– )
• COLE, VICAT (1833–1893)

See also:

• RUSSELL (FAMILY)
• RUSSELL, ISRAEL COOK (1852- )
• RUSSELL, JOHN (1745-1806)
• RUSSELL, JOHN (d. 1494)
• RUSSELL, JOHN RUSSELL, 1ST EARL (1792-1878)
• RUSSELL, JOHN SCOTT (1808–1882)
• RUSSELL, LORD WILLIAM (1639–1683)
• RUSSELL, SIR WILLIAM HOWARD
• RUSSELL, THOMAS (1762-1788)
• RUSSELL, WILLIAM CLARK (1844– )

Owing to the fact that photographic deposit scatters light more or less, the opacities measured by this plan were slightly greater than was shown when such opacities were to be used for contact printing. The final plan adopted by Abney was to place the part of the plate carrying the deposit to be measured behind a screen constructed as above. C D (fig. 4) is a Cyy/yypO~Jj~~ nl whch card be with an aperture cut j / n it which may be of any desired shape. A B in aperture was covered with trans- See also:

• PARENT, SIMON NAPOLEON (1855– )

I is a continuous spectrum taken with the electric light; no. 7 is an impressed continuous spectrum ; no. 8 shows the bright lines of metals; no. 3 the See also:

• LINE

Measurement of the Rapidity of a Plate.—The first attempt that was made to ascertain the rapidity of a plate was by Abney (Phil. See also:

• HAG

Soc. Chem. Ind. See also:

• JAN

Presumably, therefore, some degree of sensitiveness would still be preserved even at the absolute zero. Effect of Small Intensities of Light on a Sensitive Salt.3—When a plate is exposed for a certain time to a light of given intensity, it is commonly said to have received so much exposure (E). If the time be altered, and the intensity of the light also, so that the exposure (time X intensity) is the same, it was usually accepted that the See also:

• ENERGY (from the Gr. ivipyela; iv, in, ipyov, work)

1 Abney, Proc. Roy. Soc. 1893, and Journ. Camera See also:

• CLUB (connected with " clump ")

As it is, they show diminishing percentages as the light intensity is diminished. o saws set t intensities of Light Thus, when the intensity of the light is reduced to -614- of the original, and the time of exposure is prolonged 64 times, the useful energy expended on a See also:

• LANTERN (an adaptation of the Fr. lanterne from Lat. lanterna or laterna, supposed to be from Gr. Aa urritp, a torch or lamp, Aaµaew, to shine, cf. " lamp "; the 16th- and 17th-century form " lanthorn " is due to a mistaken derivation from " horn," as a m

This is the See also:

• REVERSE

Effect of Monochromatic Light of Varying Wave-lengths on a Sensitive Salt.—It has been a subject of investigation as to whether the gradation on a plate is altered when exposures are made to lights of different colours; that is to say, whether the shades of tone in a negative of a white object illuminated by, say, a red light, would be the same as those in the negative if illuminated by a blue light. Abney I announced that the gradation was different; and, quite independently, Chapman Jones made a general deduction for isochromatic plates that, except with a certain developer, the gradation was steeper (that is, the curve shown graphically would be steeper) the greater the wave-lengths of the light to which the sensitive salt was subjected. For plates made with the ordinary haloid salts of silver Chapman Jones's deduction requires modification. When monochromatic light from the spectrum is employed, it is found that the gradation increases with wave-lengths of light which are less, and also with those which are greater, than the light whose wave-lengths has a maximum effect on the sensitive salt experimented with. Thus with bromo-iodide of silver the maxi-mum effect produced by the spectrum is close to the blue lithium line, and the gradation of the plate illuminated with that light is less steep than when the light is spectrum violet, green, yellow or red. From the red to the yellow the gradation is much the steepest. Whether these results have any practical bearing on ordinary photographic exposures is not settled, but that they must have some decided effect on the accuracy of three-colour work for the production of pictures in approximately natural colours is undoubted, and they may have a direct influence on the determination of star magnitudes by means of photography. See also:

• REPRODUCTION

—Another investigation was made as to the effect of very intense light on sensitive surfaces. In this case a screen of step-by-step graduated opacities was made use of, and plates exposed through it to the action of lights markedly differing in intensity, one being that of the amyl acetate lamp, another that of the arc light, and a third the light emitted from the spark of a Wimshurst See also:

• MACHINE
• MACHINE (through Fr. from Lat. form machina of Gr. µnxavil)

If, then, we can on a photographic plate produce a developed image of the spectrum which exactly corresponds in opacity and position to the amount of red stimulation excited in those regions, we shall, on See also:

• ILLUMINATING

This gives a sensitometer of equal red values. A coloured screen has to be found which, when placed in front of the lens, will cause the opacities of the deposit on the plate, corresponding to each square of glass, to be the same throughout. This is done by trial, the colour being altered till the proper result is obtained. In a similar way the " green " and " blue " screens are determined. Coloured pigments rotating on a disk can also be employed, as indicated in the See also:

• PARAGRAPH

Sanger-Shepherd has a " one-exposure " camera by which the three images are thrown side by side on the plate. Thus any movement in the picture affects all three negatives alike. Abney has also introduced a " one-exposure " camera which takes in a larger angle than that of Sanger-Shepherd. The next point is the exposures which should be given through each screen. This can be done by placing in front of the plate and extending its whole length a scale of gradation through which the light coming from a sun-illuminated white card passes, as well as through the screens. In the case of the three-exposure camera the times of exposure are varied till the densities of the image of the gradation appear the same in each of the three images. In the case of the one-exposure camera, the light reaching the plate through the screens is altered by cutting off with a shutter more or less of the lens used. As the plates employed for the purpose of the three-colour negatives must be sensitive to every colour, the ordinary dark-room light should be most cautiously used. If used at all, it should be very feeble and development must be carried out in a dish with a cover to it. The plate is manipulated in the usual way. Joly's Process.—See also:

• PROFESSOR (the Latin noun formed from the verb profiteri, to declare publicly, to acknowledge, profess)

A " taking " screen was ruled with alternating orange, blue-green and blue lines z o u to iy in. apart, touching one another and following one another in the above order. When such a screen was placed in front of a sensitive plate in the camera, and exposure made to the image of a coloured object, there were practically three negatives on the same plate, each being confined to the area occupied by lines of the same colour. The shades of colour and the depth of the colours used in ruling depended on the See also:

• BRAND, JOHN (1744-1806)
• BRAND, SIR JOHN HENRY (1823-1888)

Such a layer is embedded on the surface of a glass plate in a waterproof vehicle, and a film of sensitive emulsion held in situ in some material, the composition of which has not been published, covers this layer. When such a plate is placed in the camera, with the back of the plate next the lens, the light passes through the coloured granules, and again we have three negatives on one plate, but instead of each negative being represented by lines as in the Joly process they are represented by dots of silver deposit. Owing to the way in which the three-coloured film is prepared, it is evident that a positive taken from such a negative could not be backed with granules of the right colour; as the granules are placed at See also:

• RANDOM (older forms randon, randrun; from the French, cf. randir, to run quickly, impetuously; generally taken to be of Teutonic origin and connected with Ger. Rand, edge, brim, the idea being possibly of a brimming river)

This he effected by printing each of the three negatives, produced for his triple See also:

• PROJECTION

The value of the exports of photographic materials from the See also:

• UNITED

L. See also:

• CHEVALIER, ALBERT (1861– )
• CHEVALIER, MICHEL (1806-1879)
• CHEVALIER, ULYSSE (1841– )

Fr. von See also:

• KOBELL, WOLFGANG XAVER FRANZ, BARON VON (1803-1882)

Zenger (1875) and D. See also:

• MACH, ERNST (1838– )

P. de Seguier and others for daguerreotype. The first record of them in England is, apparently, J. See also:

• ATKINSON, EDWARD (1827—1905)
• ATKINSON, SIR HARRY ALBERT (1831—1892)

They have since been made in various patterns, conical, oblong and square, by P. See also:

• MEAGHER, THOMAS FRANCIS (1823–1867)

G. Edwards (1855), T. Scaife (Pistolgraph, 1858), A. Bertsch (186o), T. Ottewill (1861), and others, but it was not until rapid gelatin dry plates were available in 1881 that T. See also:

• BOLAS (plural of Span. bola, ball)

Loisseau and J. B. 1 Germeuil-Bonnaud patented an opera glass camera. Various forms of portable See also:

• MAGAZINE

Loman's (1889) and R. Krugener's (1891) were early examples of the hand camera type, but great improvements have since been made. Modern cameras differ so much in details of improved construction that only a few of the more important requirements can be noticed. A camera should be well and strongly made of seasoned wood or of metal, perfectly rigid when set up, to avoid any shifting of the axis of the lens in respect to the sensitive plate. The front and back of the camera should normally be See also:

• VERTICAL (from Lat. vertex, highest point)

These swing-movements should preferably be See also:

• ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

Such scales should be verified by measurement. In hand cameras with fixed infinity focus, the necessary adjustments for distance of near objects are made on the lens See also:

• MOUNT, WILLIAM SIDNEY (1807-1868)

The image seen in the finders should correspond exactly with that on the plate. When the rising front is used special arrangements have to be made to ensure the See also:

• CORRESPONDENCE
• CORRESPONDENCE (from med. scholastic Lat. correspondentia, corrcspondere, compounded of Lat. cum, with, and respond ere, to answer; cf. Fr. correspondance)

1). Roll-holders are also a convenient way of carrying sensitive celluloid films in lengths of six or twelve exposures, rolled on spools, which can be changed in daylight. Changing boxes for holding a. reserve of plates or celluloid films in sheaths, are used with some magazine and other cameras. They are arranged to fit on the camera in place of the dark-slide and the plates are changed automatically so that exposed plates are placed in order successively at the back, a fresh plate going forward for exposure and the number of the exposure being recorded at the same time. Studio cameras, for See also:

• PORTRAITURE

Portable and field cameras include cameras of the Hare and Meagher types for outdoor work and general purposes on plates 15 in. X 12 in. to 81 in. X 61 in., and in lighter forms from 61 in. X 41 in. to 44 in. X 31 in. For general purposes they are usually made with square bellows and folding tail-board, rather more substantially than those with conical bellows intended for outdoor work. There are many patterns, the principal modern improvements in field cameras being swinging fronts, See also:

• TRIPOD (Gr. rpiTovr, Lat. tripus)

2). Most of them are fitted with focusing glass and finders, and may focus by scale in the same way as hand cameras. With an ap- 2.—See also:

• SINCLAIR
• SINCLAIR, SIR JOHN, BART

The focusing screen is superseded or supplemented by finders arranged to show the limits of the subject on the plate, the focus being adjusted by the infinity catches and focusing scales above noticed. Swing-backs and fronts are.often dispensed with, but are desirable adjuncts, and a rising and falling front particularly so. Lenses of fairly large aperture, f/6 to f/8, and good covering power, preferably of the anastigmatic type, or a rapid aplanat, should be used, but for very rapid work anastigmats working from f/4 to f/6 will be more useful. Hand cameras can also be fitted with telephoto objectives of large aperture. Some cheap hand cameras are fitted with single landscape lenses or aplanats working about fill or lower, but the want of intensity limits their use to well-illuminated subjects. Shutters of the between-lens type are now generally used in hand cameras, and for ordinary purposes should give fairly accurate exposures from i to h of a second or less and also time exposures. Some central shutters are speeded for shorter exposures to See also:

• SPA

For general work and with the modern facilities for carrying and changing plates and films in daylight, the numerous folding hand or stand cameras for plates, flat or roll films, with full adjustments, will be found most useful. Box or magazine cameras in which a See also:

• SUPPLY (through Fr. from Lat. supplere, to fill up)

Those fitted with lenses of fairly large aperture, double extension, and rising and falling fronts are to be preferred. Of box or magazine cameras there is an immense variety. In some the lens is fixed in focus for all objects within a certain distance, in others it is adjusted by a focusing scale on the lens or by an FIG. 4.—The " Deft " Folding extending front. Some have Focal-plane Camera. a single magazine, others two or more. Some take only glass plates, others plates or cut films. All of them are, however, self-contained and ready for immediate exposure. One of the earliest forms of single magazine cameras, still in use, as in the " Eureka " and " Yale," is the " bag," in Fin. 6.—The Verascope, See also:

• RICHARD
• RICHARD (d. 1184)
• RICHARD, FRANCOIS MARIE BENJAMIN (18,9-1908)
• RICHARD, HENRY (1812-1888)
• RICHARD, ST

The cameras with double magazines in which unexposed plates are kept in one recess and transferred successively after exposure to a second recess are more complicated, and many Camera. ingenious devices have been invented for effecting the change (fig. 5). Some forms are effective and popular on account of their compactness and readiness for immediate exposure, but there is always a See also:

• RISK

The light folding e wooden or aluminium stands noted below are specially suitable. Twin-lens and Reflex Cameras.—For photographing animals, objects in motion, with Twin Lenses, See also:

• SECTION
• SECTION (Lat. sectio, cutting, secare, to cut)

In order to get the greatest rapidity of exposure a focal-plane shutter is generally fitted, and by a single movement of the See also:

• RELEASE (O.Fr. reles, variant of relais, from relaisser, to release, let go, Lat. relaxare)

Io), are preferable. Al-though reflex cameras are rather heavy and bulky as hand cameras, they have many advantages over the ordinary hand camera with finder and focusing scales for the purpose of the See also:

• PRESS (through Fr. presse from Lat. pressare, frequentative of premere, to crush, squeeze, press)

Johnson's " Pantascopic " (1864), but did not come into general use till the use of curved surfaces of celluloid film enabled such cameras of convenient size and weight to be put on the market. They are on the same principle as one made by F. von See also:

• MARTENS
• MARTENS, GEORG FRIEDRICH VON (1756-1821)

The lens is fitted on a vertical axis, so that the nodal point of emergence remains motionless, and is revolved round'it by means of a handle worked by hand and carry- See also:

• ING

X 3i in., fitted with " Tes- Camera, closed. shutter, and other similar cameras which can be used for stereoscopic or single pictures. Other more elaborate See also:

• INSTRUMENTS

Jb., 1907, p. 91). The " Periphote " and Ernemann's " Rundblick " camera are improved forms (E. Jb., 1908, p. 322). Many early forms of panoramic cameras are described in B. J. A. 1892, p. 517. Colonel R. W.

See also:

• STEWART, ALEXANDER TURNEY (1803-1876)
• STEWART, BALFOUR (1828-1887)
• STEWART, CHARLES (1778–1869)
• STEWART, DUGALD (1753-1828)
• STEWART, J
• STEWART, JOHN (1749—1822)
• STEWART, JULIUS L
• STEWART, SIR DONALD MARTIN (1824–19o0)
• STEWART, SIR HERBERT (1843—1885)
• STEWART, SIR WILLIAM (c. 1540—c. 1605)
• STEWART, STUART
• STEWART, WILLIAM (c. 1480-c. 1550)

Cameras for Three-Colour Photography.—Many forms of camera have been constructed for making the three negatives required for trichromatic photography. They fall into two types: (I) those with a repeating back fitted w=:h three colour-screens or filters—red, green and violet—through which the colour impressions are made suc- cessively with one lens upon a single colour-sensitive plate, as in the Sanger-Shepherd system. The colour-screens are placed immedi- ately in front of the sensitive plate in the repeating back, which is moved on for each exposure. In a more See also:

• RECENT

E. Ives (1894) in his photo- ch*omoscope, in which three images are taken through three colour-screens on separate plates with one lens, the respective exposures being regulated by reflection of the light coming from the lens by plane mirrors on to the sensitive plates, and its filtration through the colour-screens in front of them. Many variations of this method have been proposed, in which reflecting prisms replace the mirrors. The different systems have been discussed by W. Gamble (Ph. Jour. 1905, xlv. 150), the latter also by E. T. See also:

• BUTLER
• BUTLER (or BOTELER), SAMUEL (1612–168o)
• BUTLER (through the O. Fr. bouteillier, from the Late Lat. buticularius, buticula, a bottle)
• BUTLER, ALBAN (1710-1773)
• BUTLER, BENJAMIN FRANKLIN (1818-1893)
• BUTLER, CHARLES (1750–1832)
• BUTLER, GEORGE (1774-1853)
• BUTLER, JOSEPH (1692-1752)
• BUTLER, NICHOLAS MURRAY (1862– )
• BUTLER, SAMUEL (1774-1839)
• BUTLER, SAMUEL (1835-1902)
• BUTLER, SIR WILLIAM FRANCIS (1838– )
• BUTLER, WILLIAM ARCHER (1814-1848)

Abney has described three-colour cameras for landscape work in Ph. Jour. 1904, xliv. 81, and 1908, xlviii. 331. Enlarging Cameras.—These cameras vary in form, according to the nature of the illumination, but ordinarily consist of a double or triple extension bellows camera, with a holder for the negative or transparency at one end, and for the sensitive plate or paper at the other, the lens being placed on a fixed See also:

• PARTITION

The films used are similar to the ordinary celluloid films, but in narrow bands from 1I in. to 21 in. in width, the length varying with the number of exposures required, at the rate of 16 to 20 per second. They are perforated on both sides, so that they may run true and have the necessary intermittent motion, the perforations fitting on to studs on a sprocket See also:

• WHEEL (0. Eng. hweol, hweohl, &c., cognate with Icel. hjol, Dan. hiul, the Indo-European root is seen in Sanskrit chakra, Gr. r in Aos, circle, whence " cycle ")
• WHEEL, BREAKING ON THE

W. See also:

• WOLF
• WOLF (Canis lupus)
• WOLF, FRIEDRICH AUGUST (1759-1824)
• WOLF, HUGO (1860-1903)
• WOLF, JOSEPH (1820-1899)

Soc. Arts, 1908, 57, No. 2926). Special cameras are made for various branches of scientific See also:

• RESEARCH (O. Fr. recerche, from recercher, re- and cercer, mod. chercher, to search; Late Lat. circare, to go round in a circle, to explore)

The See also:

• OBJECTIVE, or OBJECT GLASS

With the single meniscus view lens, used with its concave surface towards the object and a diaphragm in front, a square will appear See also:

• BARREL (a word of uncertain origin common to Romance languages; the Celtic forms, as in the Gaelic baraill, are derived from the English)

In order, therefore, to obtain in-creased rapidity combined with perfect definition, some means had to be found of constructing photographic objectives with larger effective apertures. This necessity had long been recognized and met by many of the best makers for objectives of the single meniscus and aplanatic types, but with only partial success, because such objectives are dependent upon the diaphragm for the further correction necessary to obtain good definition over an extended field. The difficulty was in the removal of astigmatism and curvature of the field, which, as J. Petzval had shown, was impossible with the old optical See also:

• FLINT
• FLINT, AUSTIN (1812-1886)
• FLINT, or FLTNTSHTRE (sir Gallestr)
• FLINT, ROBERT (1838- )
• FLINT, TIMOTHY (1780-1840)

3. Symmetrical doublets. 4. Triple combinations. 5. Anastigmatic combinations—symmetrical and unsymmetrical. 6. Telephotographic objectives. 7. Anachromatic combinations. They are also sometimes classified according to their rapidity, as expressed by their effective apertures, into " extra rapid," with apertures larger than f/6; " rapid," with apertures from f/6 to f/8; " slow," with apertures less than f/I1. Another See also:

• CLASSIFICATION (Lat. classis, a class, probably from the root cal-, cla-, as in Gr. icaMce, clamor)

Many lenses are made in series, differing in rapidity and angle of view as well as in length of focus. 1. Single Achromatic Combination or Landscape Lens.—This is the earliest form of photographic objective, evolved from W. H. Wollaston's improved single periscopic meniscus camera obscura lens (1812). It was made achromatic by Ch. Chevalier, and so used by L. J. M. Daguerre, though it required correction for chemical focus, as did the object glasses of telescopes or opera glasses first used for photography. The single landscape lens usually consists of an achromatic compound meniscus, formed of a biconvex positive crown cemented to a biconcave negative flint to secure See also:

• ACHROMATISM (Gr. a-, privative, xpiaµa, colour)

The full aperture has to be largely reduced by a diaphragm to improve definition; so it is slow, though many improved forms have been brought out. It has always been popular for pure landscape work on account of the equality of illumination over the plate, depth of focus, and the softness and brilliancy of the image owing to its thinness and freedom from reflecting surfaces. In some of its improved and " long focus " forms it is preferred by portraitists for large heads, on account of the general softness it gives when - used with large apertures. The following are some of the best-known improved objectives of this type: T. Grubb's "Aplanatic " (1857), f/15 to f/3o r•- Long Focus. (fig. 15) ; J. H. See also:

• DALLMEYER, JOHN HENRY (183o-1883)

W. Wray's Landscape Lens " (1886), f/II, is also useful for portraiture in the larger sizes at f/8. Fr. Voigtlander's " Wide-Angle Landscape Lens " (1888) i In the diagrams of lenses which follow, a uniform system of indicating the nature of the glass employed by means of the shading has been adopted. Flint glass is indicated thus: Crown glass of low refractive power thus: Crown glass of high refractive power thus: (These two are used indiscriminately in lenses made before the introduction of the new Jena glass.) Extra light flint glass thus: In most cases the front of the lens is on the right. .l f/i5, angle 90°, with great covering power and depth of focus. T. R. Dallmeyer's " Rectilinear Landscape Lens " (1888), ff14, angle 6o° (fig. 17), was of novel construction, free from distortion, brilliant in working and useful for copying. Messrs See also:

• ROSS
• ROSS, ALEXANDER (1699-1784)
• ROSS, GEORGE WILLIAM (1841- )
• ROSS, JOHN, or KOOESKOOWE (179o-1866)
• ROSS, ROBERT (1766-1814)
• ROSS, SIR HEW DALRYMPLE (1779-1868)
• ROSS, SIR JAMES CLARK (1800-1862)
• ROSS, SIR JOHN (1777-1856)

2. Unsymmetrical Doublets: Old Types.—This class includes objectives with comparatively large apertures formed of two dissimilar combinations, in most cases correcting each other, with a diaphragm between them. In some the single elements may be used independently. All the older " portrait " lenses, some of the aplanatic doublets and Fr. von Voigtlander's " Orthoscopic " Lens (1857), now disused, are of this type. Even with the present improved conditions, the portraitist working in a studio requires a quick-acting objective of large effective aperture and comparatively short focus, giving a brilliant well-defined image of near objects in different planes over a restricted field of view. The early single lenses were found to be too slow for portraiture by the daguerreotype and talbotype processes, and the efforts of opticians were directed to the problem of obtaining the maximum amount of light, together with good definition and flatness of field, and about 184o compound lenses were brought out by See also:

• ANDREW (Gr. 'AvBp4as, manly)
• ANDREW, JOHN ALBION (1818-1867)

He also fitted them with iris diaphragms. These forms were soon superseded by the compound portrait lens, calculated by J. Petzval and brought out by See also:

• ILI

H. Dallmeyer improved the correction for spherical aberration, and in his " Extra Quick Acting Portrait Lens " (186o), f/2.2, used for cinematograph work, attained greater rapidity. In the " Patent Portrait Lens " (1866), f/3, f/4 and f/6, angles 50° to 55° (fig. 20), he made great changes in the form and relative positions of the back elements, giving a flatter field and freedom from flare spot. By separating the two components of the back element more or less spherical aberration could be introduced to give softer definition and greater depth of focus. In 1875 Dr. A. Steinheil made an unsymmetrical aplanatic See also:

• POR

They were a further advance on the " Aplanats," working at larger apertures and giving better definition. This lens is interesting as the first in which astigmatism was eliminated by combining . a " crown-shaped " Jens of high refractivity, with a " flint-shaped " of lower refractivity, though made of the old glasses. In his " Rapid Antiplanet " (1893), f/6-5, angle 30°, Dr R. Steinheil improved the " Group Antiplanet " as regards astigmatism and covering power by replacing the thick back combination by a triple long-focus negative element consisting of a crown between two flints, with a heavy barium crown in the front element instead of a flint (fig. 23). Voigtlander, who originally constructed the Petzval portrait lens, improved it in 1878 and 1885, and now makes two lenses on the same principle, series I. f/3 2, angle 28°, for ordinary portraiture and projection, and series Ia., f/2 3, angle 22° (1900) for astrophotography, cinematography, &c., when intense illumination is required over a small field. Both are quite free from coma. proved Group Antiplanet. Actinic Doublet. Most of the above are portrait objectives of large aperture, but unsymmetrical doublets have also been made for landscape work. J. Goddard's " Combination Landscape Lens ".

(1859) was one of the first, and was free from distortion, gave a flat field, and could Jae used as a convertible lens. In 1864 T. Ross issued his " Actinic Doublets," modified from the Collen lens, in three series —" small angle," f/8, angle 4o° to 5o°; " ordinary angle," f/1d, angle 6o° to 75° (fig. 24); ' large angle," f/16, angle 8o° to 95 These lenses were similar to the " Globe," but unsymmetrical and more rapid. The separate elements could be used alone. Some of them were fitted with a shutter near the diaphragm. They were superseded by the " Symmetrical " lenses. 3. Symmetrical Doublets.—T his class includes objectives formed of two similar combinations of lenses, usually of the convergent meniscus form, with their concave surfaces inwards and a diaphragm between them; consequently they are rectilinear and practically free from marginal distortion. Until the introduction of anastigmatic doublets they were in general use for all purposes under the names " Aplanat," " Rectilinear," " Symmetrical," " Euryscope," &c. They are still largely used and have been improved by the use of Jena glasses in their construction. The first recorded lens of this type was Dr J.

W. Draper's combination used in 1839 for daguerreotype portraits, consisting of two double-convex lenses 4 in. diameter, with a united focus of 8 in., mounted in a tube with a diaphragm 3z in. in front. In 1841 T. See also:

• DAVIDSON, ANDREW BRUCE (1831–1902)
• DAVIDSON, JOHN (1857–1909)
• DAVIDSON, RANDALL THOMAS (1848– )
• DAVIDSON, SAMUEL (1807-1898)
• DAVIDSON, THOMAS (1817-1885)

Steinheil's "Aplanat." by C. C. See also:

• HARRISON
• HARRISON, BENJAMIN
• HARRISON, FREDERIC (1831— )
• HARRISON, JOHN (1693—1776)
• HARRISON, THOMAS (16o6-166o)
• HARRISON, THOMAS ALEXANDER (1853– )
• HARRISON, WILLIAM (1534–1593)
• HARRISON, WILLIAM HENRY (1773–1841)

G. Rodenstock's Bistigmats " are ofYthis class. J. Zentmayer made a similar unsymmetrical lens. In A. Steinheil's " Aplanat (1866) the same principle was carried out with achromatized 1egses, and a great Portrait Lens. APPARATUS) improvement was effected in the construction of non-distorting objectives of fairly large aperture. It consisted of two positive cemented flint menisci, each composed of a dense flint with negative focus outside and a light flint with positive focus inside, its concave surfaces facing the centre (fig. 26). This use of flint glasses alone was peculiar, former achromatic lenses having been made of flint and crown. These lenses were made in three rapidities: " Ordinary," f/6 or f/7, angle 6o°; " Landscape," f/12 to f/15, angle 90°, also used in convertible sets; " Wide Angle Landscape," f/2o to f/25, angle 104°; " Wide Angle Reproduction," similar to the last, but with sharper definition. The " Aplanat " had many advantages over previous doublets and the triplet, being more rapid, perfectly symmetrical, so that there was no necessity for turning them when enlarging, and free from distortion or flare.

There was no chemical focus. Each component could be used alone for landscape work with double focus, subject to the ordinary defects of single lenses. By the use of Jena glasses in the "Universal Aplanat" (1886) the components of this lens were brought closer together, its intensity increased, and it was made more portable. J. H. Dallmeyer had been working in the same direction simultaneously with Steinheil, and in 1866 brought out his " Wide Angle Rectilinear," f/15, angle loo°, made of flint and crown, the front element being larger than the back (fig. 27). It was slow for ordinary purposes and was succeeded in 1867 by the well-known " Rapid Rectilinear," f/8, on the same Rectilinear Lens. principle as Steinheil's " Aplanat, but made of flint and crown (fig. 28). Ross's " Rapid " and " Portable Symmetrical " lenses, Voigtlander's " Euryscopes," and other similar lenses of British and foreign manufacture are of the same type, and still in use. They are excellent for general purposes and copying, but astigmatism is always present, and although they can be used with larger apertures than the triplets they displaced, they require stopping down to secure good marginal definition over the size of plate they are said to cover.

By the use of Jena glasses they have been improved to work at larger apertures, and some are made with triple cemented elements. 4. Triple Combinations: Old Types.—This class comprises objectives composed of three separate combinations of glasses widely separated from each other. An early form of this type was made by Andrew Ross (1841) for W. H. Fox Talbot, others by F. S. See also:

• ARCHER, WILLIAM (1856— )

H. Dallmeyer's ' Triple Achromatic Lens " (1861), f/1o, angle 6o°, now out of date, was an excellent non-distorting lens, very useful for general work and copying (fig. 29). As made by Dallmeyer, the inner surfaces of the front and back components were slightly See also:

• CON

Between 1887 and 1889 the first attempts to make anastigmatic objectives with the new glasses were made by M. Mittenzwei of See also:

• ZWICKAU

Jour., 16, p. 276). It was a symmetrical doublet of novel construction, each element con- sisting of a piano-convex crown of high refrac- tivity cemented to a piano-concave flint of lower refractivity, but about equal or higher dispersion. Both the 509' uncemented surfaces were spherical and concentric. At f/16 it gave sharp definition and flatness of field with freedom from astigmatism, distortion or flare over an angle of 75°. It was an excellent lens, though slow, and has been superseded by the " Homocentric " and other more rapid anastigmats. Dr Paul See also:

• RUDOLPH
• RUDOLPH (d. 1o8o)
• RUDOLPH, or RAOUL

Jb., 1891 and 1893; M. von Rohr's Geschichte, and O. Lummer, Photo-graphic Optics, for further details). They were issued by Messrs Zeiss and their licencees (in England, Messrs Ross), in 189o, in two different types. The more rapid had five lenses (fig. 31), two of ordinary glasses in the front normal achromat, and three in the back abnormal achromat, two crowns of very high refractive power, with a negative flint of very low refractive power between them. Series VI. Series VIa. The fifth lens assisted in removing spherical aberrations of higher orders with large apertures. The second type, series IIla., f/9, 1899 (fig. 32), had only two lenses, the functions of which were as above. These combinations could not be used separately as single lenses. They are now issued as " Protars," series IIa., f/8; IIIa., f/9; V., f/18.

In 1891 Dr Rudolph devoted himself to perfecting the single landscape lens, and constructed on the same principle a single combination of three lenses, the central one having a refractive index between the indices of the two others, and one of its cemented surfaces diverging, while the other was converging. At f/14.5 this lens gave an anastigmatically flat image with freedom from spherical aberration on or off the axis. It was, however, not brought out till 1893, as a convertible lens or " Satz-Anastigmat," series VI., f/14.5, and VIa., f/7.7 (figs. 33 and 34). In the meantime Dr E. von Hoegh (C. B. Goerz) and Dr A. Steinheil had also been working at the problem and had independently calculated lenses similar to Rudolph's, but, whereas he had devoted himself to perfecting the single lens, they sought more perfect correction by combining two single anastigmatic lenses to form a doublet. Dr Rudolph had had the same idea, but Messrs Goerz secured the priority of patent in 1892, and in 1893 brought out their " Double Anastigmat," now known as Ross-Goerz " Dagor." Series III. Ross-Goerz. Series IV. Dagor." It was the first symmetrical anastigmat which combined freedom from astigmatism with flatness of field and great covering power at the large aperture of f/7.7 (fig.

35). Both these types of Zeiss's " Protars and Goerz's " Dagor " anastigmats have since been made by Messrs Ross in England. Messrs Steinheil brought out their first " Orthostigmats " in 1893, but, owing to patent difficulties, were unable to manufacture them in Germany, and they were issued later in France and England. They were followed by a second type, which has since been issued in several series by Messrs Steinheil and by Messrs See also:

• BECK (or BEEK), DAVID (1621—1656)
• BECK, CHRISTIAN DANIEL (1757-1832)
• BECK, JAKOB SIGISMUND (1761-1840)

spherical correction the separating surface should be convex towards the higher refracting medium. A fully corrected cemented lens cannot, therefore, be made with less than three glasses, but with uncemented lenses an air-space may form one of the media. In 1895 Dr D. See also:

• KAEMPFER, ENGELBRECHT (1651-1716)

Dallmeyer, was the most perfectly corrected single lens that had been Series VII. f/12.5. Series VIIa. f/6.3. evolved up to 1900, Dr Rudolph having succeeded in obtaining freedom from spherical aberration and astigmatism, and also in eliminating coma (Ph. Jour. 1901, 25, p. 68). These lenses were issued in 1895 as series VII. singly and VIIa., in combinations now known as " Convertible Protars," and the earlier series VI. and Vla. were withdrawn. The single lenses of series VII., f/12.5, angle 85°, have great anastigmatic flatness of field and only very slight marginal distortion, a condition not realized before in a single lens. The relative rapidities of the double combinations of series VIIa. vary from f/6'3 to f/8, according to the lenses used. They are excellent lenses for all general purposes. In their " Convertible Protars," series IV. (1908), f/12.5, angle 6o°, Messrs Zeiss have simplified and cheapened the construction of these lenses by the use of new Jena glasses, so that they consist of three instead of four lens elements cemented together, while possessing the same high efficiency as series VII. They are issued as " single " or " double " Protars, f/12.5 and f/6.3 or f/7, also in sets of three or four objectives of different foci, which are combined to give pictures of different angles of view from the same standpoint.

With both series when using the " Protar " lens singly, it should be screwed behind the iris diaphragm of the mount, to avoid curvature of the field, and when two such lenses are combined the one with the greater focal length should be placed in front. In 1895 Messrs Goerz patented a double anastigmat, f/5'6, with quintuple single lens components as a convertible lens, for which greater sharpness of definition and intensity, with perfect freedom from astigmatism and distortion in the single lens, were claimed. It was issued in 1898, but, like an earlier analogous quintuplet of Messrs See also:

• TURNER, CHARLES (1773–1857)
• TURNER, JOSEPH MALLORD WILLIAM (1775–1851)
• TURNER, NAT (1800-1831)
• TURNER, SHARON (1768-1847)
• TURNER, SIR JAMES (1615–1686)
• TURNER, WILLIAM (d. 1568)

Jour. 1895, 19, p. 64). Series III., f/6.5 (fig. 40), and series IV., f/5.6, are portrait lenses. In the larger objectives of series II. the back lenses are adjustable for uniform sharp definition or a soft See also:

• DIFFUSION (from the Lat. diffundere; dis-, asunder, and fundere, to pour out)

Jour., 1896, 2o, p. 117). It also approaches the triplet construction and depends on the introduction of air-spaces between the component lenses. According to Aldis, three conditions must be observed to obtain a flat field free from marginal astigmatism: (I) The convergin lenses must be of high, the diverging of low, refractive index; (2) the converging and diverging components must be separated by a considerable interval; (3) thick meniscus glasses should be used. The first " Stigmatic " was a portrait lens, series I., 1896, f/4. It has been made in two forms, first with a triple front lens, and a back negative system formed of a single thick crown lens of high refractivity with a negative ce- mented meniscus. In the second form (fig. 41) the front component consists of a cemented positive and negative, and both parts of the back component are cemented lenses. All the converging lenses are FIG. 41.-Stigmatic Portrait Lens. of dense baryta crown, Series I. while both the diverging lenses in the back component are a light silicate crown. It is fully corrected for spherical and chromatic aberration, free from distortion and nearly so for astigmatism, giving equal illumination over a flat field of 6o°.

Diffusion of focus is obtained by unscrewing the back See also:

• CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

The single components are not convertible. In 1897 Messrs Zeiss issued the " Planar," an objective of large aperture based on the principle of the See also:

• GAUSS, KARL FRIEDRICH (1777-1855)

44). The separate halves cannot be used as single lenses, neither being fully corrected for colour. It is well adapted for portraiture, groups or landscapes, especially for rapid hand camera work, on account of its covering power, with freedom from astigmatism and sharp definition with large relative aperture. In 1898 Messrs Goerz patented their " Double Anastigmat Celor," series Ib., f/4.5 to f/5.5. It is a symmetrical doublet, each element consisting of two thin single lenses: a positive of high and a negative of low refractive index, separated by an air-space (fig. 45). It is derived from the 'triple anastigmats by decreasing the refractive power of the central convex meniscus to the refractive power of air, so that it becomes a convex air-space between a double convex and a double concave lens. Less deeply curved surfaces can be given to the lenses, and the doublet gives anastigmatic flatness of field over an angle of 62° to 66°, equal to the best anastigmats with a still larger aperture. Series Ic., f/6.3, is similar and recommended for hand cameras, the aperture being smaller. Goerz's " Hypergon," (1900) f/22, angle 135°, is a Alethar," series V. (1903),f/11, is a lens with diminished secondary spectrum, for three-colour reproductions, half-tone process work, and general purposes. It is a symmetrical doublet, each element consisting of a negative and positive separated by an air-space (fig.

46). The negative is composed of three cemented lenses, which correct the spherical and chromatic aberrations more fully than hitherto possible, so that all the colours of the spectrum are focused in the same invariable plane. It gives great crispness of definition at full aperture (W. See also:

• ZSCHOKKE, JOHANN HEINRICH DANIEL (1771-1848)

In 1902 H. L. Aldis issued the Aldis Lens," f/6, a doublet composed of a cemented meniscus in front and a single double-convex back lens. It is a long-focus objective with short back focus, and is made in two forms, series II, f/6 (fig. 47), and series III. (1903), f/7.7 (fig. 48). In the latter the back element is very thin, and the front combination of See also:

• INFINITE (from Lat. in, not, finis, end or limit; cf. findere, to deave)

The " Trio " (1908) is similar, but only increases the focus one and a half times and is thus more suitable for cameras of short extension. The Aldis " Oxys " anastigmat, series II. (1908), f/5.65, angle 85°, is an improved form. Being an unsymmetrical cemented doublet it is free from the defects incidental to air-spaces and is constructed to give more perfect correction for flatness of field with large aperture and wide angle. It is generally stated that it is impossible to make a spherically, chromatically and anastigmatically corrected photographic objective with the old optical glasses. K. See also:

• MARTIN (Martinus)
• MARTIN, BON LOUIS HENRI (1810-1883)
• MARTIN, CLAUD (1735-1800)
• MARTIN, FRANCOIS XAVIER (1762-1846)
• MARTIN, HOMER DODGE (1836-1897)
• MARTIN, JOHN (1789-1854)
• MARTIN, LUTHER (1748-1826)
• MARTIN, SIR THEODORE (1816-1909)
• MARTIN, SIR WILLIAM FANSHAWE (1801–1895)
• MARTIN, ST (c. 316-400)
• MARTIN, WILLIAM (1767-1810)

It is a symmetrical Series III. Homocentric." doublet, each element consisting of a negative flint meniscus of higher refraction, and a positive crown of lower refraction with an air-space between them in the form of a negative lens. The back element can be used alone. The " Lumar " series, by G. Roden-stock, is similar. In 1902 Messrs Ross brought out the " Homo-centric," a symmetrical doublet, each element consisting of a negative and positive meniscus separated by an air-space (fig. 5o). It is constructed so that all rays of light emanating from any one point of the object are converged again into one point in the image. It is also quite free from spherical zones, is not altered in focus with different diaphragms, and thus has exquisite defining power. The colour correction is so perfect that the different coloured images are identical in size and position, thus rendering it specially suitable for three-colour and process work. The back lens can be used alone, with diaphragms, as a single lens of about double the focus of the doublet. It is made in several series: II., f/5.6, and III., f/6.3, for rapid and instantaneous work; V., f/8, for ordinary purposes; VI., f/8, for process work and three-colour reproduction.

A later series, IV. (1907), " Compound Homocentric," f/6.8, differs from the others in being a symmetrical doublet composed of two triple cemented elements, very close together and separated by a diaphragm. It is " Tessar." " Heliar." specially suitable for outdoor work, also for copying and enlarging, having good covering power. Zeiss's " Tessar " (1902) is a rapid unsymmetrical doublet, formed of two separated uncemented positive and negative lenses in the front element and a cemented meniscus at the back (fig. 51). The two halves cannot be used separately. The glasses used are very transparent, permanent and lessen the secondary spectrum. Three series are made by Messrs Ross, Ic., f/3.5 for cinematographic work and portraiture, and f/4.5 for hand-camera work and portraiture; IIb., f/6.3 for general purposes, and VIII., the " Apochromatic Tessar," specially corrected for three-colour work and reproduction. They all give fine definition over a large flat field, free from any zonal aberration. The f/3.5 portrait lenses, with double the field and covering power of the Petzval lens, are anastigmatic and free from distortion. Messrs Voigtlander's Heliar." (1902), f/4.5, angle 5o°, calculated by Dr H. Harting, is an objective of large aperture, suitable for portraits and very rapid instantaneous work, being well corrected for astigmatism, coma and curvature of field, with freedom from flare.

It is a triplet consisting of a central negative lens, with cemented double front and back lenses (fig. 52). The negative lenses are of light silicate flint, the two positive of the heaviest baryta crown. Besides being a rapid universal lens, it is specially suitable for half-tone process work, with a large diaphragm (E. Jb., 1903, p. 117). The " Dynar (1903), f/6, angle 60 , is of somewhat similar construction, but differs from the " Heliar " in the positive lenses of the cemented pairs being outside instead of towards the central lens. It can only be used as a whole. It is made of hard colourless Jena glasses, giving great brilliancy and uniformity of illumination over a large angle, and is specially adapted for very rapid hand-camera work. Dr R. Steinheil's Unofocal " (1903), f/4.5 is a symmetrical doublet, each element consisting of two single separated lenses of equal refractive power and of equal focus of opposite signs, hence its name. Each half can be used as a single lens with small stops.

In its construction a quite new principle was followed, the separation of the lenses fulfilling an important part in the colour correction, as explained by See also:

• CONRAD (d. 955)
• CONRAD, JOSEPH (1856– )
• CONRAD, or KONRAD (M. H. Ger. Kuonrdl, i.e. " keen in counsel," Lat. Conradus, It. Corrado, cf. the A.S. Ccenred)

The single elements are fully corrected astigmatically and chromatically, and can be used singly at double the focus (E. Jb. 1904, p. 35). Beck's Isostigmar " (1907) is a new anastigmat showing a dis- tinct departure from the ordinary principles of construction, in that it does not fulfil the Petzval condition that the sum of the focal powers of its individual lenses multiplied by the reciprocals of their respective refractive indices should be equal to zero, or E(1/µ0 =o. It is a 5-lens combination, two separated thin single lenses in the front element and three in the back (fig. 55). In departing from the Petzval condition very low power lenses can be used, thus reducing the initial errors to be corrected; no indi- vidual component having a shorter focal length than one-half that of a complete objective. A special feature is the excellent correction of the oblique spherical aberrations and central aber- rations, giving a practically flat field without astigmatism over angles from 6o° to 90°. The half combinations can also be used alone with diaphragms as long focus lenses of different foci (Ph. Journ. 1907, 47, p.

191). It is issued in six series: I. (1908), f/4.5, large aperture, series, for reflex press work and portraiture; Ia., f/6.5, angle 6o°-65°, latter is very useful when an extended use of the rising front is required, either at a wide or ordinary angle. V. (1908), flit, " Process " lenses specially corrected to give a flat field for copying. They can be fitted with suitable reversing prisms. VI. (1908), fJ/5.6, variable portrait lenses, adjustable for sharp or soft definition from the back of the camera while focusing. The above represent the principal types of anastigmats, but many more objectives of the kind, triple or quadruple, cemented or uncemented, with air-spaces, in many modifications, have been issued by See also:

• ENGLISH

Enlarged images of this kind can also be made by a combination of two con-verging lenses, one of them forming an image of the object, which is received on the other of shorter focus and projected on the sensitive plate, being enlarged more or less according to the optical conditions and relative positions of the lenses and sensitive plate. The .photo-heliographs at See also:

• GREENWICH

1611). See also:

• CHRISTOPHER, SAINT (Christophorus, Christoferus)

I. Porro seems to have been the first to use a combination of this kind for photographing an eclipse in 1857, and later for terrestrial objects. It consisted of a small achromatic single lens combined with a concave lens. Many attempts were afterwards made in France, and also in England, to utilize the principle, but special lenses for the purpose were not available. Ad. Steinheil constructed one in 1889 for the Brussels See also:

• OBSERVATORY

They are exceedingly valuable for obtaining details of inaccessible objects at a distance, whether architectural or topographical, and for photographing animals without approaching them too closely. Large portraits can also be taken with much better perspective effects and more conveniently than by using long-focus lenses much nearer to the sitter. With the very perfect telephoto- 4- 55 Beck's " Isostigmar." L3 vz 4... ---- -i----.--i. F- a. L • ............ ......... °----L---- A L J ~..~. . r~.w r -' as s. a, ..IFt 1:1 rw long focus, for portraiture, &c.; II., f/5.8, angle 70°, for general use, graphic objectives now available the loss of intensity of illumination. III. f/7.7, angle 65°, similar to II. but less rapid; IV. f/6.3, angle 90°; which no doubt was the See also:

• BAR
• BAR (0. Fr. barre, Late Lat. barra, origin unknown)
• BAR, CONFEDERATION OF
• BAR, FRANCOIS DE (1538-1606)
• BAR, THE

Petzval, having a positive front and negative back meniscus, with their concave surfaces facing inwards (fig. 58). As in the old " Orthoscopic ' and lenses of that type there is some outward distortion, but it is very slight. These lenses are made in five sizes with foci from 8 to 22 in., requiring camera extensions from 44 in. to 1 I is in. They magnify about twice. According to K. Martin, a telephoto-combination of the Bis-Telar type can be used in a reversed position as a projecting lens for the lantern, with the advantage of increasing the source of light (E. Jb. 1908, p. 46). Captain See also:

• OWEN, JOHN (1616-1683)
• OWEN, JOHN [OVENUS Or AUDOENUS] (c. 1560-1622)
• OWEN, ROBERT (1771-1858)
• OWEN, SIR HUGH (1804-1881)
• OWEN, SIR RICHARD (1804–1892)

In 1908 Messrs Zeiss issued their " Special Tele-objectives " in two sizes working at f/Io, the larger with an aperture of 3.14 in. and 32 in. focal length fitted in a special " tele-camera " for plates 9 X,12 cm. with a monocular field glass magnifying four times as finder. The smaller one, with 18 in. focus, is adapted for hand cameras with 6 in. bellows extension. They consist of specially corrected positive and negative combination with a definite focal length and requiring a definite camera extension, and are specially suitable for See also:

• BALLOON

It works with great brilliancy, both elements being single glasses. It was the first of the anachromatic portrait lenses. Since 1903 Messrs C. Puyo and L. de Pulligny have been experimenting with various combinations of uncorrected lenses for producing the same effect in portrait and landscape photography by the diffusion of focus produced by chromatic aberration, and suitable lenses of this kind have recently been brought out in Paris as See also:

• LES

P. Suppl. D. Ioi). Diaphragm Apertures.—In order to regulate the intensity of the illumination by the lens, to enlarge its field, and, in the case of - the older forms of objectives, to extend the area of good marginal definition, diaphragms are used, usually with circular apertures. They are made in different ways: (1) as single metal plates, fitting into a slot in the lens tube (Waterhouse diaphragms) ; (2) Rotatory: a single plate revolving on a central axis and pierced with apertures cut to fit centrically in the opening of the lens; (3) Iris: a form of diaphragm now very generally used, and very convenient, because it can be easily adjusted as required for intermediate apertures. As a rule they are placed at the optical centre between the elements of a compound lens or in front of a single one. In order to provide a uniform system of diaphragm apertures, II with those of ordinary intensity the exposures are not unduly prolonged, and good definition can be obtained over an extended field. The optical principle on which these combinations are based is very simple, and will be understood from fig. 56. It depends mainly on the fact that in order that a real image may be thrown on the screen of an object AB, the rays proceeding from it, which pass through the positive system L,, must come to a focus at a point f within the secondary focus f" of the negative system L2. Falling within this limit, they will be intercepted by L2 and made less convergent, so that instead of coming to a focus at f, they will continue to converge till they reach the screen at f", and will there form a proportionally larger image a'b' of AB than the image ab given by the positive lens alone at f; just as stated in Kepler's problem.

Moreover, this image a'b' will be of the same size as if it had been produced directly by a positive lens La with a focal length equal to lf' and this distance is the equivalent focal length of the entire system. It can be found from the formula F=f1f2/d, where fl and See also:

• F2(X2+Y2)

The tubes are adjusted so that when closed up the two foci may coincide, or nearly so, and d=o, or its minimum value; and when opened to their fullest extent the focus of the positive may fall upon the negative system, or so that d may not exceed the focal length of the negative system. Within these limits the focal length of the combination will be positive; and a real image formed on the screen. Several forms of them have been brought out by various makes, some, as Zeiss's, with a special positive lens, others for use with anastigmats and other lenses of large apertures. The negative lenses are also made of various powers. Messrs Dallmeyer's " Adon " (1902) is a telephotographic lens, for use with hand cameras, composed of two achromatic combinations adjusted for parallel rays, a front positive lens 4i in. focal length, and a back negative lens of 2;j in. focus. These are mounted to permit of great variation in the separation, so that when the Adon " is fixed on the front of a suitable lens, near or distant objects may be taken on an enlarged scale without altering the focus of the camera, or the enlargement can be varied with further extension of the camera. Used alone it is a complete telephoto ens of moderate magnifying power, and will cover plates 15 in. X 12 in. In 1903 a special form, the ' Junior Adon," was made in three kinds for use with kodaks and similar folding hand cameras, single and double extension, giving a fixed degree of magnification without loss of rapidity, while focusing can be effected by scale. It is intended to replace the front lens of an R.R. or anastigmatic lens and cannot be used independently. Messrs Busch's " Bis-Telar," f/9 (1905), is another compact fixed focus telephoto lens, specially for use with hand cameras. It is a complete lens in itself, requiring no attachments and can be fitted to a central shutter.

It is made in three sizes magnifying from two to three times. An improved form of this lens (1908), working at the large aperture of The general use of rapid dry plates and hand cameras has rendered it necessary to have some mechanical means of regulating exposures in small fractions of a second, especially for objects in rapid motion, and this instantaneous shutter has become an essential part of modern photographic equipment in many forms and patterns, but practically three types are preferred—the between-lens shutter, the roller-See also:

• BLIND, MATHILDE (1841—1896)

With a lens of 5-in. focus this would represent a displacement on the ground glass, for an object 50 ft. away, amounting to •146 in. per second, and it would require exposures between 16 and sir of a second to give maximum or minimum displacements of the image between roa and x4a of an inch. An object at the same distance moving ten times as fast would require t-to of the above exposures. If, however, the distance be increased, the possible exposure may also be increased in the same proportion, so that the object moving to m. an hour at 500 ft. distance would only require the original exposures of 116 to sr of a second. On the other hand, the limits of exposure for an object moving t m. an hour within to ft. of the lens would be betweena and A6 of a second. This is entirely See also:

• INDEPENDENT

See also:

• COVENTRY, SIR JOHN (d. 1682)
• COVENTRY, SIR WILLIAM (c. 1628-1686)
• COVENTRY, THOMAS COVENTRY, 1ST BARON (2578–1640)

Coventry as far the best. They are usually released by the pressure of the finger on the end of a lever holding the moving part in a state of tension; or better, by J. Cadett's system of pneumatic pressure, applied by means of a compressible rubber bulb and tube, which may drive a See also:

• PISTON (through Fr. from Ital. pistone or pestone, a great pestle, from Late Lat. pistare, to pound, a frequentative form of classical Lat. pinsere)

Some of them are " lateral " in their single form and " central " when double. The form and position of the effective aperture of a shutter, relatively to the lens and plate, have a strong influence, either favourable or unfavourable, on the amount of effective light passing through the lens, and its even See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

(It will be found that except in the case of the diaphragm being placed in front of the lens, the diameter of the diaphragm itself is seldom that of the effective aperture.) Every diaphragm is to be marked with its true intensity ratio as above defined, but the present intensity ratios are retained in their order of sequence: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8,f/I1.3, f/16, f/22.6, f/32, f/45•2, f/64, &c., each diaphragm requiring double the exposure required by the preceding one. In other cases apertures are to be made in uniformity with the scale, with the exception of the highest intensity, e.g. a lens of f/6.3 would be marked for f/6.3, f/8, &c. The corresponding numbers are known as f numbers, but are only applicable for a lens focused for distance. Other systems of notation are in use, but the above is generally adopted (see Fabre, T.E.P. Suppl. C. 38). Special diaphragms are in use for process work with ruled screens (see N.S. Amstutz, Handbook of Photo-engraving, 1907). See also:

• STANDARDS

322). Instantaneous Shutters. APPARATUS] what is more important, how far they can be depended on for regularity. There are many simple ways in which the actual time of exposure from opening to closing can be ascertained sufficiently closely for practical purposes. They depend upon the measurement of the trace left on a sensitive plate by the passage of a brightly illuminated object revolving at a known speed et- falling vertically through a known distance, when photographed with different speeds of the shutter against a dark background. These, and the more elaborate methods for obtaining more accurate determinations of the shutter-exposure periods and of the corresponding effective exposures —i.e. showing the actual effect of the shutter through its different phases from opening to closing—have been described by Sir William Abney in the work already mentioned, by A. Londe in La Photographie modern and La Photographie instantanee. An apparatus for testing shutters at the See also:

• NATIONAL

59 and 6o) is a good example of the type. W. Watson's " Silent " Single-flap Shutter. Double-flap Shutter. shutter is hemispherical in form and collapsible, the two wings opening out and folding together, when actuated by a special " Antinous " release, and R. & J. Beck's is another form, a single lifting flap with pneumatic release. 2. Drop Shutters.—The old simple drop shutter, in which a plate having an opening in it falls in front of the lens aperture, has been superseded by the more compact and quicker-working roller-blind shutters, which act on much the same principle. It had a theoretical interest in connexion with the effect of different forms of aperture—circular, square, or elongated—used with shutters of the lateral type, but it is now generally recognized that a more or less extended rectangular opening, of at least the full width of the lens aperture, is best for securing the even See also:

• ADMISSION

Combined Drop and Flap Shutters.—'.n early dry-plate days several forms of this kind of shutter were brought out, under the names of See also:

• PHOENIX
• PHOENIX (Gr. 4oivt )

Roller-Blind Shutters.—For general use the well-known roller-blind shutter of the single lateral type, as made by See also:

• THORNTON, HENRY (1760—1815)
• THORNTON, ROBERT (1623-1678)
• THORNTON, WILLIAM THOMAS (1813-188o)

62.—Thornton-Pickard Roller-Blind is, Rubber r in g Shutter with automatic exposure appliance. adapter. an opaque black curtain with an elongated rectangular aperture is unrolled from the lower roller on to the upper one, and held by a coiled spring on the lower roller (fig. 63). Pressure on a pneumatic bulb inflates a second smaller bulb, raising a lever which ;releases the spring, and thus brings the blind down with a rapidity which can be adjusted by turning a handle actuating the spring, the corresponding speed being shown on an indicator. For time exposures, pressure on the bulb opens the shutter, and another pressure closes it, but an arrangement is now made by which time exposures of 4, , I, 2, 3 ;seconds can be given automatically, the pressure of the bulb opening the shutter, which closes of itself at the expiration of the exposure required. The theory of shutters of this type has been very fully discussed by Coventry (op. cit. p. 5o), who shows that for any given tension of the spring the actual exposure decreases as the size of the lens aperture diminishes, while the effective exposure remains constant for all apertures. This is peculiar to the lateral shutter. He also shows that with plates of very different rapidities, though the exposure may be the same, the actual exposure effective is less with the rapid plate and a small stop than with the slow plate and a large stop; consequently the blur due to the movement of the object would be proportionately less on the rapid plate than on the slow one. Also that for any given lens the smaller the shutter the more rapid the exposure can be made, though with the same lens a larger shutter is capable of giving a more efficient though less rapid exposure. It is better, therefore, for moderate exposures, to have a larger shutter than the size of the lens requires. Sir William Abney had given diagrams of the action of a shutter of this kind in his book referred to; they show clearly that the centre of the plate gets more exposure than the margins; but practically this is not very noticeable, and the action is very regular.

6. Focal Plane Shutters.—These are also roller-blind shutters with mechanism similar to the foregoing, but arranged so that the slit in the curtain may move rapidly close in front of the sensitive plate, exposing different portions of it in turn, the intensity of the exposure being regulated by the width of the slit, whether adjustable or not, and the rapidity with which it is moved by the unwinding of a spring. The advantages of these shutters are now being fully appreciated, the principal being that they are quite independent of the lens, so that one shutter will serve for different lenses, and any suitable lens may be used at its full intensity, without the loss of efficiency inherent in the ordinary forms of lens-shutters. They thus add effectively, if not actually, to the speed of a slow lens, or if a lens be stopped down there is less loss of efficiency, with a gain in increased depth and definition. They are particularly well adapted for the very short exposures required in photographing near and quickly moving objects, racing horses, See also:

• DIVERS

The theory of these shutters is discussed by Coventry (op. cit. p. 69), more fully by Fabre (T. E. P. Suppl., C. p. 128), and their practical use in Focal Plane Photography (" Photo-See also:

• MINIATURE

The sensitive plate will be most evenly and strongly illuminated when the leaves of the shutter work inside the lens near the diaphragm, as in Bausch and Lomb's " Unicum " and other similar between-lens shutters, in general use (fig. 65). This necessitates the fitting of the lens to the shutter, but with adapters " Unicum " Shutter. " See-Saw " Shutter. it is possible to fit other lenses. Some forms are, however, suitable for use in front of the lens, such as the " Constant " and See also:

• LANCASTER
• LANCASTER, HOUSE OF
• LANCASTER, JOHN OF GAUNT
• LANCASTER, JOSEPH (1778-1838)
• LANCASTER, SIR JAMES (f1. 1591—1618)
• LANCASTER, THOMAS, EARL OF (c. 1277-1322)

A central shutter, acting as a diaphragm of variable aperture, 'eves a more efficient exposure than a lateral shutter of the same c imensions, as long as the opening is greater than the lens aperture, the coefficient of illumination of the lens varying as long as the shutter opening is smaller than that of the diaphragm used. It is desirable, there-fore, to increase the speed and use as large an aperture as possible, so that the diaphragm used may be entirely uncovered during the greater part of the exposure. 8. Iris Shutters.—These are a further development of the double curved blade central shutters, and constructed on the principle of the " Iris " diaphragm, with several leaves opening out from the centre of the lens and closing again. They are usually fitted between the lenses of double objectives, and can be made very light and compact. Theoretically this central position of the shutter is the best, and the " Iris " is the best form for ensuring the most equal distribution of light over the plate, provided, as before, that the opening is equal to the full aperture of the lens. They are made so that the periods of opening and closing may be as short as possible compared with that of full opening. They require great care in construction and fitting to the lens, and so are expensive. They can, however, be used with convertible sets of lenses of different foci fitting the same mount. Several forms are. made by British and foreign makers, with three, four or more leaves. Goerz's " Sector " shutter (fig. 67) may be taken as a type.

Georgen's " Central " shutter is very light and smooth in working, and can be used in front of a lens for telephoto work. Further details regarding the different forms of shutters, theoretical and practical, will be found in the works by Abney, Coventry, Eder, Fabre and Londe. Exposure Meters. When gelatin dry plates came into general use, and were made of many different degrees of sensitiveness, the want of a See also:

• GUIDE (in Mid. Eng. gyde, from the Fr. guide; the earlier French form was guie, English " guy," the d was due to the Italian form guida; the ultimate origin is probably Teutonic, the word being connected with the base seen in O. Eng. witan, to know)

The exposed plate was then developed and fixed, and the highest number visible indicated the rapidity of the plate. In 1890 F. Hurter and V. C. Driffield introduced an entirely new system of calculating the sensitiveness of plates of different rapidities. They make a series of exposures in seconds on different parts of the plate in geometrical progression with a standard See also:

• CANDLE (Lat. candela, from candere, to glow)

56 of the " Photo Miniature Series." The sensitiveness shown on the H. & D. scale is directly proportional to the speed number given. The method has been adopted by several dry-plate makers in 1111, ~n Ili See also:

• ILL

469, and (1900) p. 244. In 190I Chapman Jones brought out a convenient plate-See also:

• TESTER (Fr, teetiere, head-covering, from teete)

The light of the sun itself is practically the same at any given time and place year after year, but is liable to more or less local and temporary diminution by the amount of cloud, haze, dust, &c., present in the See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

Platts, F. W. See also:

• MILLS, JOHN (d. 1736)
• MILLS, ROGER QUARLES (1832– )

In Hurter and Driffield's " Actinograph " the light coefficient is given by a printed card showing the curves for every day in the year and for every hour of the day, the unit being the 1h part of the brightest possible diffused daylight when the altitude of the sun is 90°. The lens" scale shows the ratios of aperture to focal length In general use, and is calculated for single, double and triple systems of lenses. The " speed " scale is based on the exposure in seconds which with one actinograph degree of light will produce a perfect negative of an ordinaryy landscape. An additional scale is given for five different degrees of illumination—" very bright," " bright," " mean," " dull," " very dull." A table of factors for " views," " portraiture," ' interiors," " copying," is also given, and these regulate the figure to be taken for the exposure. The scales are engraved on See also:

• BOXWOOD