IO 



S. R. Williams 



is impossible to get the plates in close enough contact to the 

 wedge so as not to have thin air films intervening. This diffi- 

 culty was overcome by measuring these air films, and thereby 

 obtaining the equivalent film of air for reference. In fig. 4 is 

 shown a vertical cross-section of the cell used in getting the 

 order of interference in the wedge and an equivalent film of air. 

 W is the solid wedge between two glass plates, silvered as they 

 were in the liquid cell described above. The path of the light 

 at A gives the total thickness of air space, B that of the wedge 

 and air films on both sides. By properly silvering the lower part 

 of the wedge, the interference bands due to the wedge itself 



Fig. 4 



. Fig- 5 

 may be obtained in C. B — C gives the thickness of the air films 

 on the two sides of the wedge. A- — (B — C) gives the thickness of 



C 



an air film equivalent to the wedge. Consequently 

 gives the index as compared with air. 



A-(B-C) 

 It was impossible to get 

 the three sets of bands simultaneously, but A and B were first 

 obtained and photographed, and then by means of rack and 

 pinion the cell was raised and B and C obtained. 



With proper adjustments, there was no shift of the bands in 

 raising the cell vertically, since its sides and that of the wedge 

 are parallel in that direction. 



In solids showing double refraction, the optical system was 

 arranged as shown in fig. 5, with a nicol, A, before the slit and 

 another one, E, between the prism p., and the grating. 



r 3 8 



