REVERSED AND NON-REVERSED SPECTRA. 



115 



As b is smaller than B by equation (3), the equivalent lens is on the side of the 

 convex lens and at a distance 



behind the mirror, or 



B-b=f(f+ 2 D)/ 2 (f+D) 

 behind the concave lens. 



If the system is reversed, j\ and f% are to be replaced by j\ and /j, whereas 

 D remains positive. Thus the equations become successively 



jc_ i/(/i-0- 2 // 2 __i 



it \ i i 



I _ 2 I , 



~ 



If /i =/=/, then 



b = 



f f- 



2 D f-D 



f l -\- 2 D* 



K h f f ~ 2D 

 B - b -~J=D 



2 



Hence the equivalent lens has the same focal distance as before, but it is 

 now placed in front of the system, at a greater distance than it was formerly 

 behind it. Measured from the mirror (mirror distances, M) the data (in 

 millimeters) are roughly as follows : 



The total displacement of the equivalent lens on reversal is about i meter, 

 falling off to 96 cm. in the extreme case. The image is larger if the convex 

 lens is nearer the grating and the concave lens nearer the mirror. 



65. Effective thickness of the lenticular compensator. The compensator 

 with curved faces may change the interference pattern in two ways; viz, by 

 changing the angle of incidence and refraction of the rays at the grating, and 

 by changing the path-difference of successive rays passing through it. Both 

 conditions are virtually the same, or at least occur simultaneously. If there 

 is but one compensator, as above, the two effects must be small, since the rays 

 reflected from each of the opaque mirrors, M and N, of the interferometer, 

 must eventually enter the telescope, to unite in two nearly identical images 

 of the slit. It was rather unexpected to observe that the interferences are 

 still obtained, even when the two slit images are quite appreciably different 

 in size, but they are then confined to a single plane, as will be shown in 69. 



