1912.] 



FOR A CONCAVE GRATING. 



557 



tion from the inner surface of the colHmating objective — would be 

 done away with. Perhaps the reason Hes partly in the fact that the 

 focal length changes in passing through the spectra, so that not only 

 the inclination of the grating but also its distance from the slit 

 must be altered, and in addition the focal plane is inclined to the 

 direction of the light by an angle which varies with the setting. 

 Thus in Fig. 3 the normals to the grating and to the spectrum make 



Fig. 3. 



the same angle <^ with the light, and the distance GS between grat- 

 ing and slit is p cos 4>, where p is the radius of the grating. As the 

 inclination of the grating is altered, that of the spectrum must be 

 altered by an equal amount, and the distance GS properly changed.^ 



' Since writing the above my attention has been called to an article bj 

 A. Eagle (Astrophys. Jour., 31, 120, 1910) describing an autocoUimating 

 mounting for a concave grating. The mounting has the disadvantages men- 

 tioned, namely that the distance of the grating and inclination of the camera 

 must be separately adjusted for each angle of incidence; disadvantages which 

 it is sought to overcome in the mounting described in this paper. The ad- 

 vantages of the autocoUimating mounting are discussed at length by Mr. 

 E^gle with conclusions similar to those given here. 



