﻿The Rectification of the Spectrum. 943 



The rail BB is further provided with a slide F for the plane 

 mirror m, by which white light is thrown upon the slit c on 

 the slide S, through the condensing lens L'. L" is the 

 achromatic collimating objective, and the rod h (which may 



be grasped in front at h when necessary) is adapted for 

 clamping S and L" together, at the principal focal distance 

 of the latter. The collimator SI/' does not move with D. 

 The grating g, fixed to a horizontal disk with a spring and 

 three levelling screws, is securely fastened to the table t, 

 which here preferably rotates (not shown) with the end a of 

 the rod R. The grating, in other words, is kept normal 

 to R, so that the angle of diffraction is zero, while the angle 

 of incidence alone varies when (< passes from end to end 



