Determination of Refractive Indices 7 



tion from the bounding surfaces of the film, if the conditions of 

 reflection for the two beams are not the same. In order to 

 increase the intensity of light in reflection from the surfaces, a 

 full-silvered surface was used at the back. After repeated trials, 

 it was found that an approximate half-silvered front surface, 

 which gave a brighter image of a flame by reflection than by 

 transmission, produced the blackest bands in the channeled 

 spectra. 



O. Wiener 1 has found that the change of phase by reflection 

 varies with the thickness of the silvering, increasing as a retar- 

 dation from o to 3/4A, for all wave-lengths, as the thickness of 

 silver increased from zero to a full silver. 



The index of silver is about .25, so that in the cell that was 

 used, if the half-silvering was placed on the rear surface of the 

 front plate, and the full-silvering on the front surface of the 

 rear plate, then for both films the condition of reflecting from a 

 less refracting substance to a greater was secured for the two 

 interfering rays. 



According to Wiener, the half-silvered surface introduced a 

 retardation of about X/2, while at the rear surface it was 3A/4. 

 This made the difference in phase of the two interfering rays 

 greater by about A/4, since the ray reflected from the back sur- 

 face has been retarded that much more than the one reflected 

 from the front surface. With the order of interference that 

 was used in this work, this entered an error of less than three 

 parts in one hundred thousand in determining the order. 



In making the computation of indices, this change of phase 

 was not regarded, and since, in photographing, the bands from 

 the two films were brought into coincidence with the D x line, the 

 orders of interference for these were regarded as an odd multiple 



of^. 

 2 



(2N-\-i)\/ 2=2 fiD then becomes the formula for finding the or- 

 der of an interference band. 



The cell was housed in a pasteboard box, which kept the tem- 

 perature uniform during the photographing, so that the bands 



1 Wied. Annul . (31), 1887, p. G47. 



135 



