556 



RICHARDS— AUTOCOLLIMATING MOUNTING [April 20, 



used, the light being sent through one sHt and returned through the 

 other. The wave-length of the light which is returned through the 

 slit is given by the formula 



2,6 

 ^= sin 4>, 



III 



where e is the distance between consecutive rulings, </> the angle 

 made by the light with the grating-normal, and in the order of the 

 spectrum. It follows that at a given angle the order is twice that 

 which is produced at the center of curvature. 



Fig. 2. 



The principle of autocollimation has been often used with prism 

 spectroscopes since it was first suggested by Duboscq" and Littrow.'^ 

 It was first used with a plane grating by Liveing and Dewar,^ and 

 is employed in many recent grating spectroscopes. It has however, 

 as far as I know, not been used with the concave grating, although 

 one of the chief objections to this form of mounting — the refllec- 



" See H. Kayser, " Handbuch der Spectroscopic," I., 511. 



'O. V. Littrow, IVieii. Ber., 47, II., 26 (1863). 



* G. D. Liveing and J. Dewar, Cambridge Proc, 4, 336 (1883). 



