432 Prof. Carl Bams on 1 titer ferometry 



If 1 = 0, dl 2 =(dl) 2 . Therefore 2de / = e , dl 2 / f i 2 . In a rough 

 trial for ^' = '0226 cm., dl = '0o3 radian, ^ = 1*53, one fringe 

 reappeared. Hence 



dT$/dn=fj,de'=i x 1-53 x 27 x 10- 6 = 21 x 10" 6 cm., 



which is of the order of half the wave-length used. 



13. Inter ferometry by Displacement. — In a similar experi- 

 ment the displacement of ellipses due to the insertion of the 

 above glass, ^' = '0226 cm., was from the D line to about the 

 G line. If AN is the displacement of the mirror N, to 

 bring the centre of ellipses back to the same line, D or E, 

 we may write /i= 1 + AN//. I found 



at the E line, jjl= 1'53, 

 at the D line, /z=l*53. 



Special precautions wo aid have to be taken to further deter- 

 mine these indices. 



Thus there are two methods for measuring fi, either in 

 terms of the radial motion of the fringes, or, second, in terms 

 of the displacement of the fringes as a whole. Moreover, 

 the preceding paragraph 10 may be looked upon, reciprocally, 

 as a method for measuring d/xjdX, directly. 



Part IV. — Interferences in General and Summary. 



14. The Individual Interferences. — In figs. 5, 6, 7, gg is 

 the face of the grating, M and N the opaque mirrors, and I 

 the incident ray. 



As the result of reflexion from the top face, the available 

 air-path being y m and y' m , there must be two images of the 

 slit seen in the telescope directly, viz. a and c (fig. 5) . Of these 

 c will be more intense than a, which is tinged by the long 

 path in the glass. These two rays together, on diffraction, 

 will produce stationary interferences whose path corresponds 

 to the equation 



7)\ = 2efjb cos K. 



The optical paths of the two rays are 



reflected -refracted, I', 2y m + efi(2 cos R — sec R), 



refracted-reflected, II', 2y' m + 3e/x sec R= 2y m + ^(3 sec R- 4 sin R tan R), 



If the plate of the grating were perfectly plane parallel, 

 the slit images a and c would obviously coincide. 



The directly transmitted rays, however, after reflexion 

 from N give rise to four images of the slit. In case of a 



