282 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



conclusively that cyanin has an absorption band in the ultra- 

 violet. 



To calculate the refractive index from the photographs shown 

 on plates 21-22, it is necessary to know which pair of lines be- 

 long together. With monochromatic light it is impossible to 

 tell whether the displacement is half a fringe or one and a half, 

 or any number of whole fringes more, but if white light is em- 

 ployed and both paths are of equal length there appears a cen- 

 tral dark fringe, bordered by about a dozen colored fringes sim- 

 ilarly placed with reference to the central dark fringe. This 

 dark fringe is the only one in the whole system that can be iden- 

 tified. Substituting white light in front of slit, D, and having 

 film No. Ij on the path of the interferometer, a photograph was 

 taken. The result is shown in a in figure 7 of plate 20. When 

 looking at the fringes direct it was plain that the fringes marked 

 on the photograph belonged together. Using film No. II, a pho- 

 tograph seen in b, figure 7 (plate 20) was taken. The photo- 

 graph does not show very plainly which pair of lines go to- 

 gether, but by looking at the fringes directly with the colors 

 present those marked in the figure were easily determined to 

 be part of the same fringe. The third exposure c in the same 

 fignire was taken with film No. Ill in the interferometer. The 

 strong absorption of this film made the resulting figure hard to 

 distinguish so that even when the colors were seen directly, it 

 was difficult to tell which was the central dark fringe in the 

 part passing through the cyanin. Several observers, however, 

 independently judged the pair marked in the figure to be cor- 

 rect. 



The index of refraction can be found by the usual formula 

 for thin films: — 



nA 

 « = 1 + Te ( 33 >- 



where fi is the index for wave length X, n, the number of 

 fringes by which the central fringe is displaced, and e, the thick- 

 ness of the film. The measurement of e is difficult and the per- 

 centage error is large when a film is very thin, as is the case in 



