118 



THE INTERFEROMETRY OF 



values of i are very different in different adjustments, because the ellipses 

 may also be raised and lowered by rotating one of the opaque mirrors, as M, 

 around a horizontal axis, though in this case with rapid loss of sharpness. 

 Here the rotation of both mirrors of a pair, M' and M for instance, by the 

 same amount, is required as an equivalent to the rotation of the compensator, 

 as has been stated. The aim is to render both parallel pairs themselves parallel. 

 If x\ is eliminated from the two equations on the preceding page, 



i 2A7V cos 8/eibi 



The above data are inadequate for evaluating M, but they nevertheless 

 indicate a value entirely too low. It seems, therefore, as if some essential 

 term has been left out of sight. This is also to be inferred from the values 

 of x, which differ systematically on the two sides of the table 31. 



TABLE 32. Refraction of a glass plate. e = 0.434 cm. 

 ^ = 1.53. Rotation around vertical axis. Jamin 

 type displacement interferometer. = 45. 



go 90 100 wr nor 



In view of the discordant results obtained here and elsewhere with this type 

 of rotating compensator, the provisional parts of the apparatus were improved 

 by mounting a more accurate graduated circle with vertical axis and tangent 

 screw. A good plane-parallel plate of thickness 2 = 0.434 cm. and refractive 

 index M=i-S3 was then adjusted normal to the ray passing through it, by 

 noting the position of reversal of motion of the ellipses both when the plate 

 was rotated around a vertical and around a horizontal axis. The data of 

 table 32 and figure 79 were found while the plate was rotated on its vertical 

 axis both in a clockwise and counter-clockwise direction from the normal 

 position. In the former case (left side of curve) the normal position showed 

 the same constants before and after. In the latter this was not quite the case, 

 the micrometer being not sufficiently refined for such purposes. 



The results for ju were computed from the full equation 



AN cos 6 = e sin 2 - 



They are as good as the small values of AJV and the impossibility of obtaining 

 the zero of i with sufficient sharpness admit, and they show that the latter 

 cause adequately explains all the irregularities encountered. 



