July 26, 1912] 



SCIENCE 



127 



AN AUTOCOLLIMATING MOUNTING FOR A CONCAVE 

 GRATING* 



The usual form of mounting for a concave 

 grating is cumbersome and has the objection 

 that either the receiving apparatus or the 

 source must be movable. This may be avoided 

 by employing the method of autocoUimation, 

 using that part of the light which after being 

 diffracted is returned toward the slit. If 

 therefore the slit is on Eowland's circle, the 

 spectrum will be formed on the same circle 

 and one point of it will coincide with the slit 

 (Fig. 1). The wave-length of the light which 



Tig. 1 



is returned to the slit is given by the formula 



X^ (2e/m) sin <p, 

 where e is the distance between consecutive 

 rulings, <^ the angle made by the light with 

 the grating-normal, and m 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." 



'Abstract of a paper read before the American 

 Philosophical Society, April 20, 1912. 



' The application of autocollimation to the con- 

 cave grating was first described by A. Eagle, 

 Astrophys. Jour., 31, p. 120, 1910. 



In this method, since the focal length 

 changes in passing through the spectra, not 

 only the inclination of the grating, but also 

 its distance from the slit, must be altered. In 

 addition the focal plane, which coincides with 

 the arc of Eowland's circle, is inclined to the 

 direction of the light by the same angle as the 

 grating, and therefore the inclination of the 

 camera must be changed to correspond with 

 that of the grating. See Fig. 1, where G is 

 the grating, S the slit, and the arc at repre- 

 sents the position of the photographic plate 

 in the camera. 



Fig. 2 



In the instrument here described these ad- 

 justments are automatically made in the fol- 

 lowing manner. The grating is carried on a 

 platform which slides along a horizontal track 

 OS (Fig. 2) and is also capable of rotation 



