REVERSED AND NON-REVERSED SPECTRA. 



127 



conjugate focus of the objective of the collimator, usually somewhere 

 between the mirrors M M' and N N' in figure 73. This intersection is nearly 

 in a horizontal line, owing to the horizontal width of the collimated beam. 

 Hence as in figure 86 there are two virtual linear sources of light, a and b, nor- 

 mal to the plane of the diagram, the rays from which may be treated for 

 practical purposes as capable of interfering, since they come originally from 

 the identical slit of the collimator. The effect of rotating the compensator 

 (fig. 77) on a horizontal axis is thus to move these linear sources, a and b, 

 through each other vertically, and hence their distance apart may be called 

 h, where if d is the compensator thickness, i, r angles of incidence and re- 

 fraction at the compensator, ^ its index of refraction, 







h = d (sin i cos i tan R) =di - nearly 



M 



The fringes will thus be larger as h is smaller (fig. 86), in accordance with 

 the equation 



1-7- 



where x is the distance apart for the distance r and 6 the angle between two 

 fringes observed in the telescope, for instance. 



Experiments were made to test the last equation by attaching an ocular 

 micrometer to the telescope, so that if x is the distance and r the length of 

 the telescope, 6 is given. The distance between fringes in the same part of 

 the field was then measured from different angles of incidence i at the com- 

 pensator. The results were, if X = 6Xio~ 5 cm., r=K).$ cm. (table 34). 



TABLE 34. 



These results for 6 = \/h and = x/r may therefore be considered as iden- 

 tical, since the fringes vary in size within the field of the telescope. 



Finally, a displacement AJV at the opaque mirror will move the virtual 

 sources a and b in a horizontal plane, 2AJV cos I in the direction of rays and 

 2A7V sin I transverse to that direction if / is the angle of incidence. Hence 

 b is usually found at some point c and moves into c' by the rotation of the 

 compensator in question about a horizontal axis. The fringes do not there- 

 fore necessarily pass through infinite size unless c is at b, which would then 

 pass through a. The condition of maximum sensitiveness in transverse 

 displacement is therefore a large fringe-angle 6, a condition which requires 

 use of optic plate. Fringes subtending i would admit of Afr = 3.5Xio~ 8 cm. 



