CHAPTER XXIX 

 SPECTROSCOPIC PHOTOGRAPHY 



By G. R. Harrison 



The Spectrograph. — Any device for separating a beam of light into its component 

 wavelengths or colors is called a "spectroscope," and any spectroscope which is pro- 

 vided with a camera for photographing the spectrum which it produces is called a 

 "spectrograph." The light to be analyzed is sent into the spectrograph through a 

 slit, and a suitable optical sj^stem produces an image of this slit for each wavelength 

 present in the light beam. These slit images are called "spectrum lines," and taken 

 together they form a "line spectrum," while a plate or film on which they have been 

 photographed is called a "spectrogram." Light from certain sources like the incan- 



P 

 Fig. 1. — Component parts of spectrograph, consisting of slit S, condensing lens C, dispersing 

 prism D, camera lens E, and photographic plate P. 



descent lamp contain waves whose lengths vary continuously^ over a wide range, 

 and the resulting slit images merge together, forming a continuous spectrum. 



The usual component parts of a spectrograph (Fig. 1) are the slit S, a collimating 

 lens C or mirror to make parallel the rays of light from the slit, a dispersing element D 

 which may be a prism or a diffraction grating, and a special type of camera. This 

 camera usually contains one or more lenses E which, though uncorrected for chromatic 

 aberration, have been specially figured for sharpness of focus and flatness of field and 

 which are set to focus on the plate P the parallel bundles of light which come from the 

 dispersing element. The position of a spectrum line on the spectrogram gives a 

 measure of the wavelength of the light producing it, while the density of the image of 

 the line, when measured under controlled conditions, can be used to determine the 

 intensitj'' of the light which produced it. 



Choice of Spectrograph. — In selecting a spectrograph, the important properties to 

 be considered are the range of wavelengths over which it can be used, its dispersion, 

 its resolving power, and its speed, as determined b,v the brightness of the spectrum 

 which it produces. Secondary characteristics, such as the variation of dispersion 



804 



