SPECTROSCOPIC PHOTOGRAPHY 



807 



In order that the space between the jaws shall form a suitable line, it is necessary 

 that their edges be accurately straight and that they be mounted with their edges 

 truly parallel and with their front faces lying in the same plane. The jaws are usually 

 beveled, and placed with the beveled side turned away from the entering beam, so that 

 light reflected from them will not enter the spectrograph. 



Slits in which only one jaw is movable are cheaper than the symmetrically opening 

 type but have the disadvantage that the center of the spectrum line produced with 

 them moves when the slit width is changed. The best adjustable slits are made to 

 close at the ends only, so that their sharp jaws will not be marred by careless operaton 

 of the screw. The jaws should be made of some hard and durable material, such as 

 stainless steel, which can be ground to a 

 sharp edge and polished. 



Simple slits can be made by coating a 

 plate of quartz or other transparent material 

 with a thin opaque coating of metal or lac- 

 quer and by engraving lines of the desired 

 widths in this. Slits of several widths can 

 thus be provided, the proper one being set 

 into the slit holder as needed. For certain 

 purposes such slits are more useful than the 

 adjustable type, since a definite slit width 

 can be reproduced more accurately than by 

 setting a screw in which backlash and zero 

 position may change. 



The slit is usually mounted in a drawtube 

 in such a way that it can be moved into or 

 out of the spectroscope for focusing pur- 

 poses, or rotated about a horizontal axis so 

 that it can be brought accurately parallel 

 to the faces of the dispersing element. 

 The slit should be provided with diaphragms by means of which its length or the parts 

 of it being used can be varied. 



With stigmatic spectrographs (see page 810) an almost closed slit may 

 cause horizontal streaks to appear in the spectrum, due to dust particles which 

 close the slit entirely in spots. Such a slit should be cleaned by opening it 

 and carefully stroking its edges in one direction with a freshly sharpened stick of 

 soft wood. 



The Camera.- — In spectrographs, provision is made for holding a photographic 

 plate or film so that the spectrum is in focus throughout all parts which are simul- 

 taneously recorded. A criterion which distinguishes a good spectrograph from a poor 

 one is that, while both may give sharp lines in certain spectral regions, it will be 

 found impossible to get all parts of the spectrum into good focus at once in the poor 

 instrument. 



The plate must be bent to fit the focal curve, and it is desirable to keep this curva- 

 ture as small as possible. The plate or film is held in a plateholder which is provided 

 with templates to bend it to the proper curvature, this plateholder, in portable instru- 

 ments, being provided with a dark slide. The plateholder is in turn carried in a 

 mounting which usually has provision for moving the plateholder vertically so that 

 a number of different spectra can be photographed on the same plate. 



On account of the nonuniform dispersion of prism instruments, they are frequently 

 provided with wavelength scales, which can be impressed on the spectrogram by 



Fig. 2. — Typical slit of spectrograph. 

 The width of the slit may be varied by the 

 knurled knob and scale in the lower left- 

 hand corner of this illustration. 



