the Anomalous Dispersion of Cyanin. 43 



The interferometer method is not open to this objection 

 however, and the very close agreement between the curves 

 obtained by the two methods indicates that any possible 

 effect of the variable amplitude is too small to materially 

 influence the results obtained by the prism. 



Two of the interferometer series photographs (Plate IV.) 

 are reproduced, one obtained with a thin film, snowing the 

 displacement throughout the region of the absorption-band, 

 and the other made with a thick film, giving much greater 

 displacements, but yielding no fringes in the yellow part of 

 the spectrum. The wave-length of the light used appears 

 below each picture in the series. The two white marks 

 indicate which fringes belong together, and may be taken as 

 giving a rough measure of the displacement. The thickness 

 of the thin film was about 0*00013 millim., that of the thick 

 one '00071. One difficulty experienced in measuring the 

 retardation produced by a strongly absorbing film by means 

 of the interferometer is very troublesome, namely, that due to 

 the weakening of one of the interfering beams without a cor- 

 responding weakening of the other. This causes a strong and 

 uniformly illuminated field to be superposed on the fringes, 

 and may obliterate all traces of them. It is necessary on this 

 account to use films considerably thinner than would be neces- 

 sary under other conditions. This difficulty can probably be 

 obviated by a compensation device, and work in this direction 

 is now in progress, with every promise of success. The dis- 

 placement of the fringes can be doubled by the introduction 

 of a second cyanin film in the other interferometer path, in 

 such a position that the two films are seen side by side in the 

 half-silvered mirror, with their straight edges in contact. 

 This method was not used in the work on cyanin, but has 

 been used successfully by one of the writers in some subse- 

 quent work on the retardation caused by thin films of carbon. 

 The introduction of the compensation device will make the 

 method much more accurate, since films several times as 

 thick as those at present used will then be suitable. 



The results obtained with the interferometer are shown 

 graphically in fig. 2 (p. 44), forty observations being recorded- 

 The curve obtained with the spectrometer is shown by the 

 dotted line. 



In general the two curves agree very well, slight discrep- 

 ancies being possibly due to a difference in the optical 

 properties of fused cyanin and that obtained by evaporation. 



There is decided evidence of a rise towards the ultra- 

 violet absorption band, as there should be theoretically, 

 though we do not regard the values obtained in this part of 



