REVERSED AND NON-REVERSED SPECTRA. 



65 



ment will consist in reflecting G' in M\ and Ni, producing the planes G' \ and 

 G'i (virtual images), and then rotating M\ and G'\ 180 around IT (axis of 

 symmetry) into coincidence with N\ and G' z (interference). Then the rays 

 prolonged into a and /3 coincide with the rays prolonged into a and 8' and 

 the (virtual) diffracted rays T\ and T z become T'i and T' z . The ray on the 

 left, prolonged into s, is diffracted into T s . Then the interferences will all be 

 given by discussing the left half of this diagram, which is amplified in figure 48. 



47 



Since the distance GG r , figure 47, is very large, the rays are nearly parallel. 

 Thus the arc d'y, with its center at G, is practically a plane wave-front, 

 perpendicular to the rays in 5', /3', 7, and the diffracted rays T', T' z , and T' 3 

 are also practically parallel. Hence in the case of symmetry and coincidence 

 of Mi and Ni the points 5', /3', 7, 5', a', and s are in the same phase (diffrac- 

 tion). In other words, there is no path-difference between Y+X and Y'-}-X', 

 whether the angle of incidence is zero or not (Yi+Xi and Y\-\-X'i). The 

 whole field in the telescope must therefore show the same illumination (homo- 

 geneous light, wide slit) between the maximum brightness and complete 

 darkness. Interference fringes can occur only when the opaque mirror, MI, 

 is displaced parallel to itself out of the symmetrical position. If MI and Ni are 

 symmetrical, as in figure 47, the displacement of G', fore and aft, parallel to 

 itself, is without influence. 



This reduces the whole discussion to the normal displacements of the sys- 

 tem G', Mi, A r i, given in figure 48. Let the mirror MI be displaced over a 

 normal distance e m to the position M 3 , Ni remaining in place. Then the 

 image of G' will be at G' 3t at a perpendicular distance, e, from its original posi- 



