AN AUTOCOLLIMATING MOUNTING FOR A CONCAVE 



GRATING. 



By HORACE CLARK RICHARDS. 

 (Read April 20, 191 2.) 



" For most spectroscopic problems Rowland's concave grating is 

 an almost ideal aid," says W. Voigt in a recent article.^ Its focal 

 property enables us to dispense with lenses or mirrors, and so avoid 

 the accompanying aberration, absorption and scattering of the light, 

 and when once it is adjusted it is in focus for all orders of spectra. 

 The usual form of mounting, however, is perhaps not quite so ideal. 

 A large, perfectly dark room is required, the apparatus is heavy 

 and cumbersome, or else lacking in rigidity, and what is still more 

 important in some kinds of work, the position and direction of the 

 emergent light change with each change of wave-length. Moreover 

 it is not readily adapted to astronomical purposes. 



The theory of the Rowland mounting is well known. If the 

 source is placed at any point of the circumference of a circle con- 

 structed on the radius of the grating as a diameter, in the plane 

 perpendicular to the ruling, the spectra will all be brought to a 

 focus at points on the same circle. Of these spectra Rowland 

 selected that which was at the center of curvature of the grating as 

 giving a normal spectrum of constant scale. The necessary con- 

 ditions were insured by placing the slit at the angle of a rectangular 

 track, along the two arms of which moved the grating and the 

 camera or eyepiece, the two rigidly connected by a rod of the 

 proper length (Fig. i). It is easily seen that while the source is 

 fixed, the image is displaced in passing through the spectrum. 



To avoid this objection, Lewis^ interchanged the slit and camera, 

 and Abney^ fixed the position of the grating and camera and 



^W. Voigt, Phys. Zeits., 13, 217 (1912). 



'E. P. Lewis, Astrophys. Jour., 2, i (1895). 



'W. deW. Abney, Phil. Trans., 177, H., 457 (1886). 



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