816 HANDBOOK OF PHOTOGRAPHY 



gratings should not be cleaned with a cloth unless glass enclosed, but they should be 

 carefully stroked, parallel to the rulings, with a very soft camel's-hair brush. The slit 

 should be cleaned with a freshly sharpened stick of soft wood and should be set at a 

 nioderatel}^ narrow adjustment; its parallelism to the vertical edge of the prism or 

 grating should be checked. 



A test photograph should be taken using a mercury or iron arc, or some other source 

 having a profusion of fine lines. If satisfactory spectrograms are obtained, the lines 

 being sharp and straight, in good focus, and the spectrogram clean and free from 

 fogging and scattered light, no further adjustment is necessary. 



When a spectrograph does not function satisfactorily the following procedure will 

 l)e found useful. After the slit has been checked for parallelism to the dispersing edge, 

 it is widened to perhaps 0.5 mm. Light from a high-power mercurj^ arc is sent through 

 it, and with dispersing element and camera lens removed, the slit-coUimator distance 

 is adjusted until light of the wavelengths in a median spectral region is made into a 

 parallel beam as shown by focusing on a distant screen or wall. The prism is then 

 replaced and turned until the beam containing the same central wavelength is found 

 to be bent least, as shown by the fact that its diffuse image caught on a white card 

 (or fluorescent screen for the ultraviolet) moves first in one direction and then back as 

 the prism is rotated continuouslj^ forward. After this position of minimum deviation 

 for the central wavelength has been found, the prism is moved sideways, if necessarjs 

 so that the coUimated beam is centered on it, and is clamped in position. The camera 

 lens is then adjusted in the beam so that it is filled with light of all desired wavelengths, 

 and the camera-plate distance and plate tilt are adjusted to bring the spectrum into 

 good focus on a viewing screen held' in the plateholder. 



The final focusing adjustment can best be carried out photographically. Once 

 prism and camera lens are set, it may be found most convenient to make fine adjust- 

 ments by slightly altering the slit-coUimator distance and the plate tilt. Using a 

 narrow slit, a series of photographs is taken moving the collimator (or slit) by I'^-mm. 

 steps, and the position of best focus is marked by dots for a number of uniformly 

 spaced lines. When these dots are in a horizontal line, the collimator lens need 

 merely be set in the position indicated as best; when the line they form is straight but 

 inclined, the plateholder angle must be changed. When the}' lie on a curved line 

 the plate curvature must also be changed to compensate, though manufacturers 

 usually provide holders of the proper cur\^ature for their instruments, of course, and a 

 setting of the lenses and prisms can often be found after a number of combinations have 

 been tried, which will suit the curvature furnished. The inclined-plate technique 

 described on page 814 for focusing large gratings can often be used to advantage 

 with smaller spectrographs also. 



Comparison Spectra. — For manj;- purposes it is desirable to impress several spectra 

 on a plate without moving that plate, for even if the plateholder moves in carefully- 

 made vertical ways, some measurable lateral displacement is sure to result. With 

 stigmatic instruments a 45° reflecting comparison prism can be moved into position 

 over a portion of the slit and used to throw in a beam of light from a second source. 

 Or a Hartmann diaphragm can be used to cover parts of the slit during one exposure 

 and to uncover these parts for others. This type of diaphragm, illustrated in Fig. 10 

 together with a standard fishtail diaphragm used for varying the slit length, is designed 

 to be slipped into place over the spectrograph slit in the waj'S usually found provided 

 for this purpose. 



With astigmatic instruments it is necessary either to use some type of diaphragm 

 at the position of the external vertical focus or to place occulting diaphragms in front 

 of the plate. These should be as close to the plate as convenient, to provide sharp 

 boundaries between contiguous spectra. 



