22 THE INTERFEROMETRY OF 



of the spectra is necessary after each step of rotation. Fringes so obtained 

 are as good as those obtained by any other method. 



The range within which the fringes are sharp is small, not exceeding 2 mm. 

 of displacement of the micrometer mirror, M. A partial reason for this will 

 appear from figure 9 and results from the fact that the illumination on the 

 grating due to M moves laterally across the stationary strip due to N. Clearly 

 if the latter were also on a micrometer it might, in turn, be displaced relatively 

 to the direction of M and restore the fringes to full brilliancy. The range 

 in this case may be increased till either illuminated strip gets beyond the 

 edges of the grating. This test will presently be made. 



If the prism angle is <p and the angle of diffraction for normal incidence is 6, 

 the angle d, between the incident and reflected ray at M, is 



d = 6 p 



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 dis- 

 placement x on the grating is 



x = 2e sin (5/2)/cos 6 



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, <p may be com- 

 puted as 



C sin 6 2\c 

 sin <p = --= 

 o b 



for the spectra are in the second order. 

 The data are: 



58.Q3 cm.; 6 = 38.0 cm. ; (7 = 20.4 cm.; > = 2ooXicr 6 cm. 



Whence 



0, = i8i2' = 3 66' 5 = i 7 54'; 5/2=8 57' 



Hence 



25X0.1556 



x = - - = 0.090 cm. 



0.808 



if = 0.25 cm., as found. Thus the rays of the same origin, or rays capable 

 of interfering, are found in a vertical strip on the grating, not more than i mm. 

 wide. It is interesting to note that the fringes vanish by becoming coarser 

 and wider, corresponding to the narrowing of effective edges in contact. 



The attempt to produce these fringes with homogeneous (sodium) light and 

 a wide slit again failed, although much time was spent in the endeavor. Even 

 with a narrow slit and accentuated sodium lines (impregnated arc) the phe- 

 nomenon may be produced between the doublets, however close together, but 

 it fails to appear with the same adjustment when two corresponding lines 

 coincide. I was only able to produce it in a continuous spectrum, between 

 the two doublets and with a fine slit. It is very important to ascertain the 

 reason. 



