C. Barus — Use of Compensators. 301 



o is smaller than B by equation (3), the equivalent lens is on 

 the side of the convex lens and at a distance 



B- M=(f-2D>)/2(f+ D) 



behind the mirror, or 



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



behind the concave lens. 



If the system is reversed, f and f 3 are to be replaced by 

 — f and — f\ whereas D remains positive. Hence the equiva- 

 lent 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. The total displacement of the equivalent 

 lens on reversal is about one meter. 



3. The effective thickness of the lenticular compensator. — 

 The compensator with curved faces may change the interfer- 

 ence 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 JV, 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. They 

 are then confined to a single plane, however, as will be shown 

 in §6. 



Since the beam of light coming out of the colimator and 

 traversing the grating is a vertical ribbon of light, several cms. 

 high, vertically, but very thin in comparison (a few mms.) hori- 

 zontally, it is relative to the vertical plane that the marked 

 effect must be expected. If the beam consists merely of the 

 axial pencil, the distortion of pattern due to the introduction 

 of the doublet is slight for any value of the distance apart 

 lenses, J). The two lenses are practically equivalent to a 

 plate. If a broad beam • is in question and the rays retrace 

 their path, the same is still true. But if on changing D the 

 rays do not retrace their path, so that the equivalent lens 

 is convergent or divergent, then the rays after leaving M 

 re-impinge on the grating at different angles than before and 

 the interference pattern is correspondingly changed, principally 

 in its vertical relations. 



Thus it is the lens system which changes the obliquity of 

 rays lying in a vertical plane and passing through the grating 



