806 HANDBOOK OF PHOTOGRAPHY 



persion obtained with a given angular dispersion can be varied by changing the focal 

 length of the camera lens. It depends also on the inclination of the focal curve to 

 the optic axis of the camera. 



The dispersion of prism spectrographs is nonuniform, depending as it does on the 

 variation of the refractive index of the prism material with wavelength, and may 

 vary tenfold along one short spectrogram. Prisms deviate the short waves more than 

 the longer, and the dispersion increases as the wavelength decreases. 



Resolving Power. — The ability of a spectrograph to separate close-lying lines is 

 measured by its resolving power. Resolving power P is defined as \/d\, d\ being 

 the wavelength difference between two lines which the instrument can just resolve 

 at wavelength X. If unresolved, the two lines will appear as one. 



Most spectrographs have theoretical resolving-power values lying between 2000 

 and 200,000, the lower value being typical for a small prism instrument and the higher 

 for a large diffraction grating. The theoretical limiting resolving power of a given 

 spectrograph increases as the linear aperture of its camera lens is increased (if this is 

 kept filled with light) and as the angular dispersion of its prism or grating is increased. 

 By using good optical parts it is possible to approach the theoretical resolving power 

 very closely. 



Dispersion and resolving power are closely related but are two distinct properties 

 of a spectrograph and should not be confused. Increased linear dispersion may 

 separate the centers of two close spectrum lines more widely, but since, if unaccom- 

 panied by an increase in resolution or of angular dispersion, it results merely in 

 increased magnification, the lines are no more clearly resolved than before. 



The ability of a spectrograph to resolve two close lines may be reduced by its 

 auxiliary parts. When properly designed, a spectrograph will be provided with a 

 camera of such focal length that the resolving power \/d\ of the instrument will 

 match, through the linear dispersion dX/dl, the resolving power \/dl of the photo- 

 graphic materials used to record the spectrum, unless some other consideration such 

 as speed intervenes. 



Speed. — The efficiency of a spectrograph is commonlj^ defined as its speed times 

 the purity of the spectrum it produces. Speed depends on the intensity of the trans- 

 mitted light, which will in turn depend on the fraction of incident intensity of each 

 wavelength which penetrates the instrument, and on the numerical aperture of the 

 camera lens. The total light flux which can be obtained from any spectrograph, 

 assuming that a light source of the extent required to fill the slit used is available, is 

 limited by the size of slit which can be used with a given purity of spectrum. This 

 means that the larger a spectrograph is made, if its proportions are kept the same, the 

 more light flux can be sent through it. With a given spectrograph, shortening the 

 focal length of the camera lens will increase the light intensity, the spectrum lines 

 being correspondingly reduced in size. 



Most prism spectrographs operate at numerical apertures of from //12 to //24, 

 while grating instruments may have apertures as low as //70. Exposure times 

 required may range from a few seconds to several hours, depending on the speed of the 

 instrument and of the photographic materials used and the intensity and energy dis- 

 tribution of the source being photographed. 



The Slit. — Since spectrum lines are merely images of the slit, this slit is one of the 

 most important parts of a spectrograph. It should be carefully made and adjusted, 

 with its edges kept clean and smooth. The opening of the slit should be variable in 

 width, preferably between 1 and 0.002 mm. The slit jaws are usually drawn apart 

 by a calibrated screw acting in opposition to a spring which tends to move the jaws 

 together. A tj^pical slit is shown in Fig. 2. 



