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HANDBOOK OF PHOTOGRAPHY 



slit is used. In this way the focal curve can be located within 1 in. or less either 

 way. An eyepiece can be used in the visible region to approximate stUl more closely 

 to the focal curve, the slit being greatly narrowed for this purpose. The remainder 

 of the focusing process must be carried out photographically. 



A number of small brackets are hung on the camera, each holding a small piece 

 of photographic plate inclined at an angle of 45° to the vertical, the center of each 

 plate lying in the focal plane of the camera, as in Fig. 7. Photographs are taken 

 with these inclined plates distributed at intervals, and the spot of best focus is marked 



on each plate. The camera is then moved to 

 the position corresponding to best focus and 

 inclined test plates are taken again but at 

 intermediate points. This process is re- 

 peated until the whole of the camera is in 

 satisfactory focus. 



Illumination of the Spectrograph. — Vari- 

 ous methods are available for directing into 

 the spectrograph the light to be studied, 

 that most commonly used being one in which 

 an image of the source is focused on the slit 

 by means of a double-convex lens. Either 

 one of the two conjugate foci of this lens can 

 be used, the first giving a diminished and 

 the second an enlarged image. The latter 

 — Use of inclined photographic jg generallv to be preferred when the source 

 determining position of best -^ ^f ^^^^ extent, 



Fig. 7 

 plate for 

 focus. 



is of small extent, provided the angular 

 aperture of the lens is sufficient to fill the 

 collimator of the spectrograph with light under these conditions. This is illustrated 

 in Fig. 8, from which it will be seen that use of an extra-large collecting lens Li is 

 of no value if it more than fills the angular aperture of the spectrograph collimator. 

 In certain cases one can place the source directly in front of the slit in such a way 

 that the spectrograph is filled with light. Under such circumstances, as in a third 

 method (Fig. 9) in which a lens is placed directly in front of the slit in such a way as to 

 throw an image of the source on the collimator lens, unless the slit is narrow the lens 

 may not be filled with coherent radiation, and a loss in resolving power may result. 



Fig. 8. — Diagram illustrating illumination of spectrograph. The light source A, and lens 

 L, illuminate the slit iS, the collimating lens C, and the dispersing element D. 



At a certain optimum slit width, given approximately by the formula W /\ = N.A., 

 where W is the slit width, X is the wavelength of the light considered, and N.A. is the 

 numerical aperture of the collimator, the line intensity reaches a maximum, though 

 the intensity of the continuous spectrum continues to increase as the slit is widened. 

 For this reason a narrow slit may help to reduce continuous background relative to the 

 line spectrum. 



In considering the efficiency of any method of spectrograph illumination, it 

 should be kept in mind that so long as the entrance slit and the collimator lens or other 



