SPECTROSCOPIC PHOTOGRAPHY 



813 



Concave Grating Mountings. — If a diffraction 

 grating be ruled on a concave spherical mirror, no 

 collimator or camera lenses are required, for light 

 sent through a slit placed anywhere on a circle 

 whose diameter equals the radius of curvature of 

 the mirror and passes through the mirror will be 

 brought to focus on this circle as a spectrum. 

 The focal curve is known as the "Rowland circle." 

 Since so many possible relative dispositions of slit, 

 grating, and camera are possible, numerous special 

 mountings have been designed for special purposes. 



Table I gives the advantages of the standard 

 mountings for concave gratings. 



Small commercial concave-grating spectro- 

 graphs, which are of necessity portable, ordinarily 

 use the Eagle or in some cases the Rowland mount- 

 ing. When one is faced with the necessity of 

 choosing the most suitable mounting for a large 

 grating, the first consideration must be that of the 

 space available. Where only a long narrow corridor 

 or a small vertical shaft can be used, the Eagle 

 mounting is useful. The cross section of its con- 

 taining box is conditioned only by the length of 

 spectrum which it is desired to photograph at one 

 setting, and by the baffles needed to cut down 

 stray light. 



If a room of medium size (say 12 by 15 ft.) is 

 available, a 21-ft. grating in the Wadsworth mount- 

 ing will probably be found more useful than a 

 shorter focus grating in any other mounting. This 

 is because the grating of longer radius will cost no 

 more than a shorter one for the same area of rul- 

 ing, and the advantages of a stigmatic mounting 

 are obtained, while the grating can always be used 

 at its full dispersion in another mounting. 



The Paschen mounting is so much more flexible 

 than others that its use is advantageous in many 

 cases where sufficient space is available. For a 

 21-ft. grating the loom should be at least 25 bj'- 

 15 ft. for a half circle or 30 by 25 ft. for a full 

 circle, with a separate room to serve as a source 

 room, the slit being mounted on a pier and let 

 through the partition between the two rooms. 

 Detailed directions for adjusting gratings in the 

 Rowland and Abney mountings will be found in 

 Kayser's "Handbuch der Spektroskopie," while 

 the original article by Eagle should be consulted in 

 connection with his mounting. 



When the camera of a grating spectrograph 

 has been placed in approximately the proper posi- 

 tion, it is clamped in the position of best focus b,v 

 viewing the spectrum lines produced on a piece of 

 white paper or a fluorescent screen when a broad 



