Non-reversed Spectrum Inlerferometry . 



415 



exceeding 2 millimeters of displacement of the micrometer 

 mirror, M. A partial reason for this will appear from fig. 9 

 and results from the fact that the illumination on the grating 

 due to M, moves laterally across the stationary strip due to JV. 

 Clearly if the latter is also on a micrometer, it might, in turn, 

 be displaced in the opposite direction to M and restore the 

 fringes to full brilliancy. The range in this case may be in- 



Fig. 9. 



creased, till either illuminated strip gets beyond the edges of 

 the grating. 



If the prism angle is $ and the angle of diffraction for normal 

 incidence is 0, the angle, 8, between the incident and reflected 

 ray at Jjfis 



8 — 6- <f> 



Thus e tan 8/2 is the displacement of the strip of light on the 

 mirror M, if e is the normal displacement of the latter. Hence 

 the corresponding displacement, x, on the grating is x = 2e 

 sin (8/2)/cos 0. 



If b be the distance from the prism to the light spot reflected 

 on M, and c the distance from there to the bright spot on the 

 grating, <f> may be computed as 



. , 2Ac 

 sin^» = — — 



for the spectra are in the second order. 

 The data are : 



10 e A = 58-93 cra ; b — 38-0 cm ; c= 20'4 cm ; D — 200 X 10" 



cm. 



