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Spectroscope

slit, lens, jaws, spectrum, collimator, focus, length, aperture and telescope

SPECTROSCOPE (Fr., Spectroscope ; Ger., Spektroskop) An instrument designed for seeing a spectrum and consisting essentially of three parts—the slit, the dispersing medium, and the observing telescope. The latter is essential if it is desired to see the spectrum uncontaminated by white light, though it is possible to see a spectrum in any ordinary room with no more apparatus than an opaque card with a slit in it and the dispers ing medium. If, for instance, a slit about long and 1- in. wide is cut in an opaque card about 12 in. long and 6 in. wide, and this card is placed on the cross-bar of a window, a fairly pure spectrum can be seen by going to the other side of the room and examining the slit through the dispersing medium held close to the eye. A complete spectroscope is shown at A, in which C is the collimator, consisting of a slit s at the focus of the lens i ; P is the prism, or dispersing medium ; and F the telescope which receives the refracted and dispersed beam. A third tele scope is sometimes provided which throws a numbered scale on to the second face of the prism, whence it is reflected to the telescope and the eyepiece.

The slit is usually made of metal, and con sists of two plates, one of which may be fixed and the other movable, the latter being actuated by a micrometer screw so that its distance from the fixed jaw may be varied at will. In purchas ing a slit, stipulation should. be made that the jaws are of platinoid, a hard white alloy which does not readily tarnish. It is advisable also to have what is known as a symmetrical slit—that is, one in which both jaws are moved simultan eously by one screw. For very accurate wave length determination this is important, as it is the centre of the line which is measured, and with a symmetrical slit this is constant, whereas with an unsymmetrical slit the spectral line widens on one side. On the other hand, it is quite possible to obtain wave-length readings, of suf ficient accuracy for all ordinary work, with a slit having only one movable' jaw if the edge of the spectrum line that coincides with the fixed jaw is taken for measurement. It is neces sary that the jaws of the slit be bevelled with the bevel inside, and that the bevel be blackened, as this prevents reflection of the light. The jaws must be parallel and without any side slip, and parallelism of the jaws can be easily tested by opening the slit, then gradually closing it down till the aperture entirely disappears. This it should do throughout its entire length simul taneously. The edges of the jaws are easily damaged, and therefore the slit should never be screwed up roughly or too tightly. As the slit for normal work is extremely narrow, dust particles are very apt to lodge between them and give rise to dust lines, which are longi tudinal black lines running throughout the length of the spectrum. These dust lines, whilst

detracting from the appearance of the spectrum, are convenient in one way, as they prove whether the slit is parallel to the edge of the prism or the ruling of the grating, as if parallelism exists the lines are parallel to the edges of the spec trum. To clean a slit from dust particles d soft wood match stick should be cut to a fine bevel or chisel shape, and, the jaws being opened, the match should be inserted and the jaws closed till just gripping the wood, and then the match should be moved up and down and the jaws again opened and the match removed. When chemicals or solutions are burnt or vola tilised by a spark dose to the slit, it is apt to get splashed with particles of the salt ; it is advisable then to use a condenser, which enables the spark to be worked some distance from the slit and focuses the image on the slit, or a thin microscopic cover glass may be temporarily placed in front of the slit.

Frequently a slit is provided with draw plates which cover parts of the slit, thus enabling two or more contiguous spectra to be photo graphed.

The dimensions of the slit depend upon the aperture of the collimator lens and the prism or grating, and it may be taken as a safe rule that the clear aperture of the slit should not be less than one-third the diameter of the lens. To some slits is also fitted a piece of metal with a wedge-shaped aperture cut in it ; this is used to limit the size of the spectrum, and is an advantage for spectrography, as it enables one to narrow the width of the spectrum and thus prevents the diffusion of stray light. In most spectroscopes the aperture of the slit is narrowed by an internal diaphragm, which prevents scat tered light reaching the collimator lens. It is usual, too, for the slit to be mounted on a focus ing tube, but for spectrography it is just as well to determine once for all the accurate dis tance of the slit from the collimator lens, and fix it so ; and this does not at all interfere with the use of the instrument for visual work. The focal length of the collimator lens is merely a question of convenience, though naturally there are certain theoretical considerations that govern this. The rays from the slit are parallel ised by the collimator lens, and we may calcu late the breadth of the slit image as formed by the telescope or camera lens thus : let . = the height or length of the slit, f = the focus of the collimator lens, F = the focus of the telescope or camera lens, and S = the image formed by the latter, then S = s x 7 For instance, suppose the length of the slit = 12 mm., the collimator lens focus = z so mm., and the camera lens focus = so° mm., then— soo