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SENSITIVE PLATES, FILMS AND
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SENSITIVE PLATES, FILMS AND PAPERS Sensitive Dry Plates.—A See also:special feature of See also:modern See also:photography is the use of trustworthy ready-prepared sensitive dry plates and films in different grades of sensitiveness, so that there is nc See also:necessity for the photographer to prepare his own plates, nor, indeed, could he do so with any See also:advantage. The practice of outdoor and studio photography has thus been very greatly simplified; and although with wet See also:collodion there was the advantage of seeing the results at once and retaking a picture if necessary, the uncertainties connected with the use of the See also:silver See also:bath and collodion, and the amount of cumbrous apparatus necessary for preparing and developing the plates, far outweighed it. There is also an enormous saving of See also:time, in using dry plates as compared with wet, by deferring development. In tropical climates, also, dry plates can be used when See also:work with wet plates would be impossible. On the other See also:hand, the uncertainty of more or less See also:random exposures on ready-prepared Plates must not be overlooked. Besides their use in taking negatives, See also:gelatin dry plates are also largely used for See also:printing transparencies, See also:lantern slides, enlargements, &c. For negative work they are prepared with an emulsion in gelatin of silver bromide, alone or with the addition of silver iodide or chloride, and are to be obtained in five or six degrees of rapidity: " slow," for photo-See also:mechanical or " See also:process " work; " See also:ordinary," for See also:general purposes when See also:quick exposures are not required; " rapid," for landscape and portraits; extra rapid," for instantaneous exposures; and " See also:double extra rapid," for very quick snapshot work in dull See also:weather or for special subjects. These latter kinds are exceedingly sensitive, and require See also:great care in use to avoid See also:fog. In See also:order to prevent halation, or irregular See also:action by reflection from the back See also:surface of the See also:glass, dry plates are coated with a non-actinic " backing," which can easily be removed before development. Self-developing dry plates were introduced in 1906, in which the developing See also:agent is mixed in the film itself, as in the See also:Ilford " Amauto " See also:plate, which only requires See also:immersion in a See also:solution of washing soda for development, or, as in the See also:Wellington " Watalu plates, applied on the back of the plate, See also:plain See also:water only being required for development, this application also preventing halation. The slow plates used for printing lantern slides and transparencies are usually prepared with an emulsion of silver chloride with or without See also:free silver nitrate and other haloids. The rendering of photographic plates isochromatic or sensitive to all See also:colours by See also:dyeing them with eosin, or other suitable dyes, has been greatly improved by the use of new dyes, especially those of the isocyanin See also:group, prepared by Dr E. See also:Konig of the Hoechst factory, and known as " orthochrom T," " dicyanin," " pinaverdol," ' pinachrom " and " pinacyanol," the latter of which can confer on a silver bromide plate as high a degree of sensitiveness for red as erythrosin does for yellow; also F. Bayer's " Homocol," Dr A. Miethe's " See also:ethyl red," and other similar dyes (see E. Jb., 1905, pp. 183, 336). Panchromatic plates are now largely manufactured and used for all photographic work in which a true rendering of the relative See also:colour luminosities is essential, and more particularly for the various methods of colour See also:reproduction in which plates are required to be sensitive to red, See also:green and See also:blue-See also:violet. They are made in different degrees of general and colour sensitiveness, according to the purpose for which they are required, the ordinary " isochromatic " being most sensitive for yellow and green, and the " panchromatic " for red, See also:orange and yellow, as well as for green, blue and violet. To obtain the best results from all these plates it is necessary to See also:screen off the blue and violet rays with yellow or orange transparent screens, or colour filters, made of coloured glass, or glass coated with coloured gelatin, collodion, &c., or with glass cells containing solutions of suitable dyes or salts. For the various processes of three-colour reproduction panchromatic plates and special red, green and blue-violet filters have to be used for taking the three negatives, their intensities and absorptions being carefully adjusted to the particular plates in use; the same applies, but less strictly, to the yellow screens used with ordinary isochromatic plates. Dyes specially suitable for these colour-filters have been prepared by Dr E. Konig. Various kinds of colour screens for ordinary, microscopic and trichromatic work are made commercially, and Messrs Schott of See also:Jena make a special yellow glass in three tints for the purpose. Plates for Colour Photography.—In 1868 See also: 671). The " See also:Florence " See also:chromatic plate (1905), worked out in See also:America by J. H. Powrie and Florence M. See also:Warner, was an improvement on the Joly method, the colour screen being photographically printed on a glass plate, coated with See also:pan-chromatic emulsion and exposed to the coloured object through the screen (Penrose Pictorial See also:Annual, 1905-1906, p. III). Some See also:good results were produced, but it has not come into use. After several years of laborious See also:research, Messrs Lumiere, of See also:Lyons, adopting Ducos du Hauron's coloured See also:grain method, succeeded where he had failed, and in 1907 brought out their " Auto-chrome " plates, in a very See also:complete and See also:practical See also:form, making it possible to produce photographs in the colour of natural objects by one exposure instead of three, as in the ordinary three-colour processes. Glass plates are coated with an adhesive medium over which is spread a mixture of See also:potato See also:starch grains, of microscopic fineness, stained violet, green and orange, the interstices being filled in with See also:fine See also:carbon See also:powder to form a tricolour screen, dark by reflected and of a pinkish, pearly See also:appearance by transmitted See also:light. This is varnished and coated with a thin sensitive panchromatic emulsion of gelatino-silver bromide. The plates are exposed in the See also:camera from the back, through the tricolour films, using also a special compensating orange-yellow screen, before or behind the See also:lens, then See also:developed as usual, producing a negative coloured See also:image in the complementary colours, which is then treated and re-versed so as to produce a See also:positive coloured image by transmission, showing the picture in its proper colours. The results thus obtained are remarkably good and practically solve the problem of See also:direct colour photography in a See also:simple and fairly inexpensive manner (see Agenda Lumiere, 1909). In C. L. See also:Finlay's " See also:Thames " colour plate (1908) the tricolour screen is formed by rows of circular dots coloured alternately orange-red and green and the intermediate spaces blue. It is used alone, the coated surface being placed in contact with a panchromatic plate, the uncoated See also:side towards the lens. It carries See also:register marks for adjusting it to the finished picture after development and reversal of the image. These screens, being more transparent than the " Autochrome," require less exposure, but the colour rendering is not so perfect. In the Jougla " Omnicolore " plate (1909) the tricolour screen and sensitive surface are combined on one plate as in the " Autochrome," but the screen is made up of a See also:series of blue-violet parallel lines, with intermediate alternate broken lines of orange-red and yellowish-green at right angles to them, the red narrower than the green. The relative sizes of the coloured dots in the three plates are approximately: " Autochrome " starch grains . -Ei to ssa in. " Thames " plate, dots, See also:diameter . . sia „ " Omnicolore " plates, blue See also:line red square E. Fenske's " See also:Aurora " plate (1909) is a tricolour screen formed by coating a glass plate with a mixture of finely divided particles of gelatin, dyed orange-red, green and blue-violet, without any intervening spaces. The grain generally is coarser and more irregular than in the " Autochrome " plates, but optically corresponds more closely to them than the " Thames " or " Omnicolore screens do. These plates are issued uncoated for use with any suitable panchromatic plate. A later process is due to Dufay. With the exception of the " Autochrome,” these processes are still more or less in the experimental See also:stage. Celluloid Films.—In order to avoid the See also:weight of glass plates, which may become burdensome on a tour, and also the See also:risk of breakage of valuable records, thin films or sheets of celluloid coated with sensitive emulsions can be used, with great saving of bulk and weight and no loss of efficiency, though such films are some-times liable to deterioration by See also:long keeping before or after exposure. They are made in two thicknesses, stiff or flexible, the stiff being used exactly as plates, but held in a See also:carrier or simply backed with a card or glass plate, while the flexible are made up in separate sheaths with cardboard backing, as in the " Kodoid films, or in convenient packages of twelve or more in " film packs " of various patterns. Flexible films of this See also:kind on celluloid have for many years past also been prepared in long strips of different widths suitable for use in hand cameras of the Kodak types and in See also:roll-holders. In the See also:early forms of roll-holders the films were used alone, and being unprotected had to be changed in the dark See also:room, but, as already stated, they are now supplied on spools in cartridges which can be changed in daylight. C. Silvy seems to have been the first to employ this method in 187o. In these cartridges the film is attached to a much longer See also:strip of See also:black See also:paper, and rolled up with it, so that several turns of the paper have to be unrolled before the film is ready for exposure, this point being marked on the outside paper for the successive exposures, with See also:numbers visible through a red screen at the back of the holder. When all have been exposed, the black paper is rolled on for several turns, and when taken out of the holder the loose end is fastened up till the film is developed. As these films are principally used for landscape work, it is now usual to make them isochromatic, and they may be used with or without a yellow screen. They are also made " non-See also:curling " by being coated with gelatin on both sides. Negatives taken on these thin films have the advantage that they can be printed from either side without perceptible loss of See also:definition, which is useful in printing by the single See also:transfer carbon process, and in some of the photo-mechanical printing methods. Flexible transparent films in sheets and rolls have also been prepared upon hardened gelatin, but it is difficult to retain the See also:original dimensions of the film owing to expansion of the gelatin. Paper coated with sensitive emulsions has been successfully used for making negatives in the same way as the celluloid films, and is cheaper, but much more liable to deterioration from atmospheric action before and after exposure, and unless developed soon after exposure the impressed images may fade and become undevelopable. Such papers are, however, still used in meteorological and other self-recording See also:instruments. Stripping films of thin celluloid upon a paper support were introduced by Messrs Wellington and See also: Papers for the platinotype processes, sensitized with salts of See also:platinum and See also:iron, are also manufactured for printing out entirely or for development with potassic oxalate. Prints on these papers have the advantage of being permanent.
Messrs See also:York Schwartz and J. Mallabar's process of developing and toning prints made on a special sensitive paper prepared with an emulsion of silver phosphate was introduced by Messrs See also:Houghton in 1908 under the name of " Ensyna." Very See also:short exposures to See also:day or artificial light are required, and with a special developer (" Ensynoid ") permanent prints are obtained with a varied See also:scale of tones similar to those given by toning with gold, the colour of the print being determined by the exposure, short exposures giving See also:purple and long exposures See also: 115, 729; and further, Ph. Journ., 1900, 24, p. 221). By a series of observations he ascertained the multiplying factors of most of the developers in ordinary use, and in 1905 brought out his " factorial calculator " and a " dark-room See also:clock " for facilitating the working of the method. The former is made of See also:aluminium, and consists of two circular disks, the upper smaller one rotating and carrying a pointer. The See also:outer disk is marked with a scale of Watkins' factors for the different developers, as given in the " instructions " accompanying the See also:instrument, and is used to denote the " time of development " in minutes. The scale on the inner Calculator. room Clock. disk shows the " time of appearance " in seconds or minutes. In use the pointer is set to the See also:factor for the developer in use, and against the " time of appearance " on the inner scale will be found the See also:total number of minutes required for complete development (fig. 72). The " calculator " can be used with any ordinary clock or See also:watch, but the " dark-room clock " (fig. 73) has been specially constructed for the factorial system. It is an improvement on the earlier forms of Watkins' Eikronometer," and has a 4 in. See also:dial with so minute and 100 seconds divisions, very plain for dark rooms, centre seconds hand, stop action and outside See also:indicator to See also:mark the completed time. The seconds hand completes the revolution in Too seconds, while the minute hand does so in to minutes, or sufficient for the longest ordinary development, though it runs on, if necessary, very much longer, both hands starting together always at O. In 1908 Watkins brought out another system of " thermodevelopment " by time dependent on the use of a See also:standard " time developer," the duration of the development, at a given temperature, being modified according to the make and See also:speed of the particular plate in use. The temperature variations are indicated by a movable scale, or " thermo-calculator," on the See also:bottle of developer, the variations for development speed of various plates being given approximately on the ' Watkins' Plate Speed See also:List," which thus shows the " speed of plate " and " speed of development " with the standard developer at 6o°. This method is well adapted for plates, films and stand development in tanks or See also:machines, no observation of the plate being required, and the times are most conveniently observed with the " dark-room clock." Full details of these two distinct methods of development will be found in the 4th edition of the Watkins' See also:Manual of Exposure and Development. C. W. See also:Piper's " photographer's stop clock " (1906) is a more elaborate clock, intended for use not only in " time development " but for all photographic operations in which accurate See also:control in regard to time is of importance. It is fitted with a See also:gong and arranged to work by " time " or " bulb." Once started, by pressure on a See also:lever or on the bulb, it will continue to go until stopped, striking the gong at the completion of every minute, when the seconds hand reaches the zero point. A second pressure on the bulb stops the clock, so long as the pressure is continued, while pressure on a lever stops it permanently. It is thus useful for timing any intermittent operations, whilst the clock adds up the separate times and prevents the occurrence of errors difficult to avoid when timing with an ordinary watch. By an additional See also:attachment a prolonged time exposure with the camera may be terminated, or an " instantaneous " or short " time " exposure given at any prearranged time. Messrs Houghton's " See also:Ensign " clock for time development has a dial with 6o divisions, a single hand, and is fitted with a gong. It can be set to See also:ring an alarm See also:bell at the expiration of any See also:period from one minute to one See also:hour, can be started or stopped immediately and is easily read in the dark-room. It requires no winding up, the action of setting providing the tension for the recording movements. It can be stopped and started at will and the bell arranged to give a short or prolonged ring. S. See also:Stanley's is another convenient form, with a 41 in. dial, divided into 6o seconds and 6o minutes, the thick hand recording the seconds and the thin hand the minutes. Several forms of developing tanks and machines have been constructed for developing a number of exposed plates, together with ordinary or dilute developers, with the aid of the factorial system or independently of it. The Kodak " Automatic Developing Tank " (1905) is a useful arrangement by which bands of ex-posed roll films can be developed in daylight, without any need of a dark-room (fig. 74). The exposed film is See also:wound from the spool into a red celluloid See also:apron contained in a See also:box A, then placed in the tank B, where it is See also:left in a dilute developer for about twenty minutes, and requires no See also:attention. It gives very good results. For the " Brownie " films a special daylight developing box is made. With the Kodak " Eastman Plate-developing Tank " (1908) the exposed plates are removed, in the dark-room, from the plate holders and placed, in pairs back to back, in a special framework holding six pairs, which is lowered into a See also:metal tank containing the developer, and is fitted with a watertight lid so that it can be inverted during development. A clock See also:face, with pointer, by which the period of development may be noted is fitted outside the tank. Another apparatus of the kind is made for developing celluloid films exposed in the " Premo Film Packs " (fig. 75). Other forms are made, and in some the fixing and washing can also be effected. These tanks undoubtedly See also:save much time arid trouble in developing a large number of exposed plates or films, and have been found to work with efficiency and regularity. Eastman Kodak Co. brought out in 1907 a See also:machine for developing paper prints on bromide or gaslight papers. Additional information and CommentsThere are no comments yet for this article.
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