410 



C. Barus — Methods in Reversed and 



Fig. 7. 



through the wedge twice ohliquely. The reduction, however, 

 would here be too complicated, but will he treated later. 

 The method is interesting as allowing of the complete control 

 of a single fringe ; i. e. the equivalent of 30 X 10"° cm. As 

 this corresponds to -028 cm on the micrometer, the displacement 

 8x = -001 is equivalent to 10"" cm. Furthermore the method 

 presents an expeditious means of finding a — \/2(/u. — l)Bx 

 when a is very small. 



B. In the next place the revolving compensator, C, fig. 3, 

 was employed. This also proved to he an admirable device 

 for controlling the fringes, and it was 

 much more rapid than the preceding. 

 Unfortunately the computation is incon- 

 venient as the normal position cannot be 

 "7 / ascertained with sufficient accuracy. To 



find it, the plate was revolved until the 

 fringes changed their direction of motion. 

 This is an indication of the insertion of 

 the minimum thickness of glass, but is 

 not sharp enough for precision. Hence 

 on repetition the data are not liable to 

 be coincident. Mean values are given, 

 i denoting the angle of incidence. 

 Same plate as in the preceding work, e = "STO " 1 . 



No. of fringes passing: 







10 



20 



30 



40 



50 



Mean i : 



o u 



5-4° 



7-8° 



9-5° 



11-0° 



12-3° 



Another somewhat better and thicker plate was now inserted 

 with the following mean results. Thickness e = •489 cm . 



No. of fringes passing : 







10 



20 



30 



40 



Mean i : 



U 



5-2° 



6-8° 



8-3° 



9-6° 



The reason for the large discrepancies found is not clear to me, 

 even in consideration of the wedge-shaped plates. The mean 

 of the .results may, however, be used for computation. 



The path increment introduced by the glass of thickness 

 e = -4:89 cra and index of refraction fi = l - 526, at an angle of 

 incidence * and refraction r for n fringes, beginning at i = 

 may be written (see fig. 7, where /is the incident ray) 



(1 \ /cos (i — r) \ 

 1) —el * '-- 1) 

 cos r ) \ cos r / 



This is a cumbersome equation. If the angles i are small, the 

 cosines may be expanded and then approximately since i = fir 

 nearly, 



n\ = e(ix— 1) i' /2/x. 



