the Anomalous Dispersion of Cyanin. 37 



All previous workers have credited cyanin with but a single 

 absorption-band, the maximum of which is located not far 

 from the sodium lines, but we have found a second band in 

 the ultra-violet beoinnino- at wave-length '00037. 



Two methods have been employed, namely, spectrometer- 

 readings with cyanin prisms, and readings of the displace- 

 ments of the fringes in a Michelson interferometer produced 

 by thin films of the dye. 



A large amount of preliminary work was done on the 

 preparation of prisms. Many dyes w 7 ere tried, but cyanin 

 seemed to be the only one suitable, consequently the work 

 has been limited thus far to this single substance. The 

 difficulty of squeezing thin enough prisms to transmit yellow 

 light has been already alluded to. Outside of the absorption- 

 band it is possible to work with prisms of angles as large as 

 15 or 20 minutes. Within the absorption-band a prism of 

 1 minute angle transmits practically nothing. By using a 

 small screw press arranged to work within an air-bath kept at 

 the temperature at which cyanin was found to be most fluid, 

 we finally secured some fairly good prisms of the requisite 

 thinness, which was found to be about 30 seconds. It was 

 exceedingly difficult to split off one of the glass plates without 

 shattering the prism, but even this was successfully accom- 

 plished, for parts of some of the prisms. The method 

 employed in the determinations of the refractive indices in 

 the visible part of the spectrum was the same as that described 

 in the previous paper, the slit of the instrument being 

 illuminated with monochromatic light obtained from a large 

 direct- vision spectrometer. A portion of this light, made paral- 

 lel by the collimating lens, passed through the prism, another 

 portion passed through a strip of clear glass immediately 

 adjoining the prism. Two images thus appeared in the tele- 

 scope, an undeviated, due to the light coming through the 

 clear glass, and a deviated formed by the prism. In this 

 wav the effect of any slight prismatic form of the plate-glass 

 was eliminated. The strongly absorbed rays only pass 

 through a very narrow strip bordering the refracting edge of 

 the prism, consequently the image is much broadened by 

 diffraction, and accurate measurements are obtained with 

 great difficulty. 



In the following table are given the values of the refractive 

 indices for various w T ave-lengths obtained with prisms of 

 angles varying from 24 seconds to 17 minutes, something 

 over 80 observations in all. Each one of these observa- 

 tions is the mean calculated from several settings of the 

 spectrometer. 



