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solar image, to be mounted almost in contact with the camera slit. The spectroscope is then moved parallel to itself, admitting to the collimator slit light from all parts of the sun's disk. Thus a monochromatic image of the sun, formed of a See also:

GREAT

great number of successive images of the spectral line employed, will be built up on the plate. As the only light permitted to reach the plate is that of the calciumline, the resulting image will represent the See also:

DISTRIBUTION (Lat. distribuere, to deal out)

distribution of calcium vapour in the sun's See also:

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

atmosphere. The calcium clouds or flocculi thus recorded are invisible to the See also:

eye, and are not shown on direct solar photographs taken in the See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary way. The calcium flocculi, on See also:

account of the brilliant reversals of the H and K lines to which they give rise, and the See also:

PROTECTION

protection to the plate afforded by the diffuse dark bands in which these See also:

bright lines occur, are easily photographed with a spectroheliograph of See also:

low See also:

dispersion. In the See also:

case of narrower lines, however, higher dispersion is required to prevent the light of the continuous spectrum on either See also:

side of the dark line from blotting out the monochromatic image. A spectroheliograph which gives excellent results with the lines of calcium, See also:

HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen and See also:

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

iron is shown in the figure. This instrument, used since 1905 in See also:

CONJUNCTION (from Lat. conjungere, to join together)

conjunction with the See also:

SNOW (in O. Eng. sndw; a common Indo-European word; cf. in Teutonic languages, Ger. Schnee, Du. sneeuw; in Slavonic snieg', Lith. snegas; Gr. vi4a, Lat. nix, nivis, whence the Romanic forms, Ital. neve, Fr. neige, &c.; Ir. and Gael. sneachd; the original

Snow (See also:

HORIZONTAL

horizontal) telescope of the See also:

MOUNT, WILLIAM SIDNEY (1807-1868)

Mount See also:

Wilson Solar See also:

OBSERVATORY

Observatory, was constructed in the observatory instrument See also:

SHOP

shop in See also:

PASADENA

Pasadena.

It consists of a heavy See also:

CAST (from the verb meaning " to throw "; the word is Scand. in origin, cf. Dan. kaste, and Swed. kasta; " cast " in Middle Eng. took the place of the A.S. weor pan, cf. Ger. werf en)

cast-iron See also:

PLATFORM (Fr. plateforme, i.e. ground plan)

platform (a) mounted on four See also:

STEEL, FLORA ANNIE (1847– )

steel balls (b) which run in V guides of hardened steel. Most of the See also:

WEIGHT

weight of the instrument is floated on See also:

mercury contained in three troughs (c, c, c) which form See also:

PART

part of the cast-iron See also:

BASE

base. The platform carries the two slits, the collimator and camera objectives and the See also:

PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism-See also:

TRAIN (M. Eng. trayn or trayne, derived through Fr. from Late Lat. trahinare, to drag, draw, Lat. trahere, cf. trail, trace, ultimately from the same source)

train. An image of the sun, about 6.7 in. in See also:

DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter, is formed by the Snow telescope on the collimator slit (d). This slit is See also:

long enough (81 in.) to extend entirely across the solar image and across such prominences of ordinary height as may happen to See also:

lie at the extremities of a See also:

VERTICAL (from Lat. vertex, highest point)

vertical diameter. After passing through the slit the diverging rays fall upon the 8 in. collimator See also:

OBJECTIVE, or OBJECT GLASS

objective (e), which is constructed in the manner of a portrait See also:

lens in See also:

order to give a See also:

sharp See also:

field of sufficient diameter to include the entire solar image. In the Snow telescope the ratio of See also:

APERTURE (from Lat. aperire, to open)

aperture to focal length is 1 :3o. Hence light from any point on the slit will fill a circle about 2 in. in diameter on the collimator objective, as its focal length is 6o in. Since the diameter of the solar image is 6.7 in. there is a slight, but inappreciable loss of light from points in the image at the extremities of a vertical diameter. The rays, rendered parallel by the collimator objective, meet a plane See also:

MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror (f) of silvered See also:

glass, which reflects them to the prisms (g, g'). These are of dense See also:

flint-glass (Schott 0.102), and each has a refracting See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle of 63° 29'. Their width and height are sufficient to transmit (at the position of minimum deviation) the entire See also:

beam received from the collimator.

After being deviated 18o° from the See also:

ORIGINAL

original direction, the dispersed rays fall on the camera objective (h), which is exactly similar to the collimator objective. This forms an image of the solar spectrum in its focal plane on the camera slit (i). Beyond the camera slit, and almost in contact with it, the photographic plate-See also:

carrier (j) is mounted on a fixed support. In order to bring a spectral line upon the camera slit, the slit is widely opened and the plane mirror (f) rotated until the line is seen. A See also:

CROSS

cross-See also:

HAIR (a word common to Teutonic languages)

hair, in the focal plane of an eyepiece, is then moved horizontally until it coincides with the line in question. The slit is narrowed down to the desired width, and moved as a whole by a See also:

MICROMETER (from Gr. wcp6c, small, jArpov, a measure)

micrometer See also:

SCREW (O.E. scrue, from O. Fr. escroue, mod. ecrou; ultimate origin uncertain; the word, or a similar one, appears in Teutonic languages, cf. Ger. Schraube, Dan. skrue, but Skeat, following Diaz, finds the origin in Lat. scrobs, a ditch, hole, particularl

screw, until it coincides with the cross-hair. The eyepiece is removed and the photographic plate (k) placed in position. An electric motor, belted to a screw (1 or 1') connected with the spectroheliograph, is then started.' The screw moves the spectroheliograph at a perfectly See also:

UNIFORM

uniform See also:

RATE

rate across the fixed solar image. Thus a monochromatic image of the sun is built up on the fixed photographic plate. The spectroheliograph, originally designed for photographing the solar prominences, disclosed in its first application at the Kenwood Observatory (See also:

Chicago, 1892) a new and unexplored region of the sun's atmosphere. Photographs of the solar disk, taken with the H or K line, show extensive luminous clouds (flocculi) of calcium vapour, vastly greater in See also:

AREA

area than the sun-spots. By setting the camera slit so as to admit to the photographic plate the light of the denser calcium vapour, which lies at low levels, or that of the rarer vapour at high levels, the phenomena of various superposed regions of the atmosphere can be recorded.

The See also:

LOWER

lower and denser vapour appears as bright clouds, but the cooler vapour, at higher levels, absorbs the light from below and thus gives rise to dark clouds. The first photographs of the sun in hydrogen light were made with the spectroheliograph in 1903. These reveal dark hydrogen flocculi, which appear to lie at a level above that of the bright calcium flocculi. They also show less extensive bright flocculi, usually in the immediate neighbourhood of sunspots, and frequently eruptive in See also:

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

character. These rise 'Two screws, of different See also:

pitch, are provided, to give different speeds. from a low level, and sometimes reach considerable elevations in the form of eruptive prominences. In such an exploration of the sun's atmosphere it might be anticipated that definite currents, or some evidences of atmospheric circulation analogous to those See also:

FAMILIAR (through the Fr. familier, from Lat. familiaris, of or belonging to the familia, family)

familiar in terrestrial See also:

METEOROLOGY (Gr. JerEwpa, and hb'yos, i.e. the science of things in the air)

meteorology, would be discovered. Neither the forms nor the motions of the calcium flocculi revealed the existence of such b The Five-See also:

FOOT

foot Spectroheliograph of the Mount Wilson Solar Observatory (camera lens, camera slit and plate carrier in See also:

section). currents, but in the higher region shown by the hydrogen photographs the distribution of the dark flocculi suggested the operation of definite forces, though their nature remained obscure until the See also:

SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

spring of 1908. At that See also:

time monochromatic photographs of the sun were first made on Mount Wilson with the red (Ha) line of hydrogen, previous hydrogen photographs having been taken with H/3, Hy or HS in the See also:

BLUE (common in different forms to most European languages)

blue or See also:

VIOLET

violet. On account of the relatively great strength of Ha at a consider-able distance from the photosphere, the new photographs recorded flocculi at high levels previously unexplored. The forms of these flocculi show that all sun-spots are vortical in nature, and are probably analogous to terrestrial cyclones or tornadoes.

Most of the solar vortices indicate clockwise rotation in the See also:

SOUTHERN

southern hemisphere and See also:

COUNTER

counter-clockwise rotation in the See also:

NORTHERN

northern, as in the case of terrestrial cyclones. But frequent exceptions have been observed in which the direction of rotation is reversed. The study of these vortices has led to the See also:

DISCOVERY

discovery of a magnetic field in sun-spots, apparently caused by electric convection in the vortices. It is evident that by the use of a spectroheliograph of sufficiently high dispersion, photographs may be taken of vapours in the sun represented by lines narrower than those of calcium and hydrogen. Such See also:

WORK

work has been in progress both at Mount Wilson and at See also:

MEUDON

Meudon, and the erection of a spectroheliograph of 75 ft. focal length on Mount Wilson was at the end of 1908 contemplated for an See also:

early date. Descriptions of spectroheliographs by See also:

Hale, Deslandres, Newall and others, may be found in various papers in See also:

ASTRONOMY (from Gr. &arpov, a star, and v ,usiv, to classify or arrange)

Astronomy and See also:

ASTROPHYSICS

Astrophysics, Astrophysical See also:

Journal, Comptes rendus, Bulletin astronomique, and other See also:

PERIODICALS

periodicals. (G. E.

End of Article: SPECTROHELIOGRAPH

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