SPECTROSCOPIC PHOTOGRAPHY 805 



with wavelength, the amount of scattered light, and the sizes and shapes of the spec- 

 trum lines produced, are sometimes of importance. 



In choosing a spectrograph for any problem one should first determine whether a 

 prism or a grating instrument will be most suitable. The advantages of prism 

 instruments are their high light-gathering power (which results in greater intensity of 

 the spectnim and hence in shorter exposures when it is to be photographed), their 

 ruggedness and permanence, and the fact that they can readily be made to give 

 stigmatic spectra. They suffer from the disadvantage that their dispersion changes 

 markedly with wavelength (although much less with frequency), and they can, of 

 course, be used only in regions of the spectrum to which their optical parts are trans- 

 parent. Their fundamental dispersion and resolving power are usually smaller than 

 those which can readily be obtained with diffraction gratings. Prisms are very widely 

 used in portable instruments, for studying comparatively simple spectra, and for any 

 purpose for which relatively great light-gathering power is required. 



The diffraction grating is coming more widely into use even where only low resolv- 

 ing power and dispersion are needed, and since it is unrivaled for obtaining high values 

 of these quantities, it seems destined to play a much larger part in the development of 

 spectroscopy in the future than it has in the past. It can be used in such a waj'- as to 

 produce an almost normal spectrum, in which the dispersion is nearly uniform over the 

 length of a spectrum plate, and it requires a minimum of adjustment. 



A further advantage of the grating is that it can be used in the reflectmg concave 

 form (page 813) without any transparent optical parts; consequently a single instru- 

 ment can be used over the entire photographic range. Grating spectroscopes can be 

 obtained having a resolving power as high as 400,000 and with dispersions of as much 

 as 0.1 A. per mm. Their chief disadvantage is that most gratings throw the light not 

 into one spectrum but into several, and since usually only one of these is wanted at a 

 time, much of the light may be wasted. 



Spectral Range. — Spectrum photography in the infrared is commonly limited to 

 the range 12,000 to 7500 A., in the visible it covers the entire range 7500 to 4000 A., 

 and in the ultraviolet it goes from 4000 to 1000 A., although these limits are somewhat 

 arbitrary. 



Prisms have limited transparency, and several must be provided to cover the 

 different ranges. Flint glass, quartz, and rock-salt prisms are most frequently used 

 for the infrared, glass prisms of various sorts for the visible, and quartz or rock-salt 

 prisms for the ultraviolet down to 2000 A. Below this wavelength only fluorite or 

 lithium fluoride optical parts can be used. Fluorite is transparent to about 1250 A., 

 while lithium fluoride which will transmit to 1050 A. can occasionally be produced. 

 No suitable material has yet been found which is transparent to shorter wavelengths 

 and only the concave grating spectrograph can be used in this region. For wave- 

 lengths shorter than about 2000 A. all light paths must be in vacuum (except that dry 

 nitrogen, hydrogen, and helium can be used in parts of these regions) since the air 

 begins to absorb at this wavelength. 



The most common practice is to use glass optical parts in prism instruments 

 designed for the visible and photographic infrared and crystal quartz optical parts for 

 those photographing that part of the ultraviolet to which air is transparent. 



Dispersion. — The dispersion of a spectrograph can be measured as angular or as 

 linear dispersion. The angular dispersion dd/d\, the change in angle with wavelength 

 of the light emerging from the prism, is fundamental and depends on the dispersing 

 element. The linear dispersion is of more practical interest, as it gives the actual 

 separation of two close lines on a spectrogram. In common practice the spectral 

 range covered by 1 mm. of plate is used to measure the dispersion; thus 30 A. per mm. 

 is a low dispersion, while 1 A. per mm. is a relatively high value. The linear dis- 



