in Case of Non-reversed and of Reversed Spectra. 09 



Y, X, T, and Y, X', T, respectively ; the incident ray T at 

 an angle of incidence di, into Y„ X J5 etc., and Y/, X/, etc., 

 respectively ; both at a mean angle of diffraction dO (nearly) to 

 the right, corresponding to di. 



The angles of diffraction {di = 0) are 0„ and 2 , the double 

 angles of reflection therefore 8 = 6„ — 0„ on both sides, the 



Fig. 4. 



Ma. 3. 



double angles of the grating G' with the mirrors M l and N x 

 symmetrically, a = 6 V + 6„. 



The normal from the point of incidence at G, and at G', X 

 and n, makes angles 8/2 with Y and X, respectively, on both 

 sides. 



The method of treatment will consist in reflecting G' in M x 

 and N„ producing the planes G/ and G 2 ' (virtual images), and 

 then rotating J/, and 6r,' 180° around IT (axis of symmetry) 

 into coincidence with X x and G 2 ' (interferences). Thus the 

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

 into a' and 8' and the (virtual) diffracted rays T 1} T 2 , become 

 T/ and T„' . The ray on the left prolonged into e, is diffracted 

 into T 3 . Then the interferences will all be given by discuss- 

 ing the left half of this diagram, which is amplified in figure 4. 



Since the distance GG', figure 3, is very large, the rays are 

 nearly parallel, and hence the arc S'7 with its center at G, 



