SPECTROSCOPIC PHOTOGRAPHY 



811 



and it is advisable to keep the illuminated portion of the slit as short as 

 possible when high resolving power is required. However, since each 

 astigmatic line image has a central portion of uniform intensity whose 

 brightness depends on the length of the illuminated slit, astigmatism may 

 cause a serious decrease of intensity. Stigmatic spectrographs have the 

 advantage that with them small right-angled prisms and Hartmann 

 diaphragms can be used at the slit for introducing comparison spectra, 

 rotating sector disks or step diaphragms can be used at the slit for 

 photographic photometiy, and Fabry-Perot or other interferometers can 

 be crossed with them. Also, if the source is imaged on the slit, variations 

 in intensity along the lines produced may give useful information as to the 

 points of origin of the lines in the source. These considerations make 

 especially useful the stigmatic mounting of the concave grating, devised 

 by Wadsworth and realized by Meggers and Burns. 



Astigmatism is occasionally useful, as with certain types of inten- 

 sity measurements where uniform spectrum lines are desirable and for 

 producing spectrograms which are neat in appearance. Also it is possible 

 in certain cases to utilize the separation of the horizontal and vertical 

 focal curves to image slit and photometric device on a single plate 

 simultaneously. 



Commercial Spectrographs. — Most spectrographs manufactured com- 

 mercially are enclosed, portable units, varying in length from 18 in. to 

 8 ft. In the smaller instruments no adjustments of the prism or camera 

 are necessary, since the entire transmitted spectiiim can be obtained 

 at a single setting. In larger instruments, where the whole spectrum 

 cannot be recorded at once on a plate of reasonable size, some provision 

 must be made for turning the prism and changing the focal distance of 

 the lenses and the tilt of the plate when various regions of the spectrum 

 are to be photographed. 



The greatest number of spectrographs of any one tj^pe manufac- 

 tured are those which contain quartz optical parts. These can be 

 obtained in three or more standard sizes, the small and medium models 

 being of fixed focus. 



The small quartz spectrograph usually covers the range 8000 to 

 1850 A., with a spectrum length of about 85 mm. This type finds its 

 greatest usefulness at wave-lengths shorter than 2500 A., where its rela- 

 tively great light transmission and high aperture aid in rapid photog- 

 raphy of a difficult region where absorption and lack of plate sensitivity 

 conspire to reduce the intensity of the recorded spectrum. It can be 

 obtained fitted with a transparent wavelength scale, and a fluorescent 

 screen can be used to make the ultraviolet spectrum visible and thus 

 to aid in the preliminary focusing adjustments. 



Until recently the most commonly used spectrograph was the medium- 

 sized quartz instrument, manufactured by a number of firms. Lenses 

 of about 600-mm. focus and 50-mm. diameter are used, giving a spectrum 

 from 2100 to 8000 A. about 200-mm. long. The prism, of the Cornu type, 

 may be about 40-mm. high by 65-mm. length of face. A standard 4- 

 by 10-in. photographic plate is ordinarily used. This is the largest 

 standard size of quartz instrument which will give the entir-e ultraviolet 

 region in air at a single setting of the prism and camera. 



The medium-sized instrument can be obtained with or without a 

 transparent scale of wavelengths. The variation of dispersion with wave- 

 length is Avell illustrated by the scale shown in Fig. 4. 



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