﻿8 MORRIS-AlREY, Determination of Wave-lengths. 



Xi being the wave-length of the light which now falls at 

 the centre of the camera. 



For any other rotation, say S 2 , we have 



2d cos \ sin e = n\ 

 . cos3 1 _ X t 



COS £ 2 ^2 



AXIS OF CAMERATUBE 



AXIB OF COLLIMATOR 



Fig. 4. 



Evidently it is immaterial in which direction we rotate 

 the grating. There will be therefore two positions of the 

 grating, equally inclined to the position of symmetry in 

 which any wave-length \ can be brought to the centre of 

 the camera. The angle between the two positions is 2^, 

 and it is more convenient to measure this than B v 



The fiducial mark at the centre of the camera was 

 produced by a fine steel point fixed in the camera, whose 

 shadow appeared on every photograph. The grating was 



