66 



THE INTERFEROMETRY OF 



the same strip or region (fig. 47). They may become sharp by changing 

 the focal plane of the eye-piece. When parallel to the length of the spectrum 

 and seen in a strong telescope, about 7 lines alternately black and white may 

 be counted. The whole lie within a strip of not much above DiD z width. 

 The range of displacement of M for a rotation of 180 of fringes is about 

 0.25 cm. Change of size of fringes from red to violet is hardly appreciable. 



' 





V 



The same results may be obtained by placing a screen 5S, figure 46, with 

 two parallel slits, under the vertical telescope, to admit and limit the two 

 rays c and c' in figure 44. It was found that slits 3 or even 6 mm. apart may 

 still show fringes, though they are obviously smaller as the distance apart is 

 greater. The most interesting results were obtained by bringing the rays a 

 little beyond the edge of the prism, so that the spectra in the telescope 55', 

 figure 48, are separated at some distance. A long collimator (i meter) is 

 advantageous. In this way the character of these fringes was definitely 

 established. They are of the elliptic type, as suggested in the figure, char- 

 acteristic of the displacement interferometer, and the cases, figure 45, a, b, c, 

 d, e, are thus merely the intercepts of ellipses with the distant centers, 

 between parallels. Very coarse central fringes were obtained in the dark 

 gap, and the displacement at mirror M, between the extreme hair-line types, 

 was now only about 0.08 cm., all fringes filing by the coincident D lines 

 while the micrometer shifted. 



Hence this method is available for displacement interferometry, the hori- 

 zontal type c, figure 45, normal to the D lines, being used for setting the mi- 

 crometer at M . If a plate of glass of thickness E and index of refraction n 

 is inserted normally into one beam, the corresponding air-path is 





when B is Cauchy's constant and the wave-length X. I assumed B = 4.6 X icr 11 

 and .B/X 2 = o.o265. On the other hand, the same air-path for a normal dis- 

 placement, e, of the mirror as given by the micrometer-screw is 



X = 2e cos (90 0)/2 = 20cos 8/2 



where 6 is the angle of diffraction of the grating G, figure 43, and b b' is nor- 

 mal to the direction of incident light. Hence 



E(M-i + 2/X 2 ) = 2ecos (go-0)/2 

 A rough experiment was made with a plate E = 2.2 cm. thick, ^=1.53. 



