the Anomalous Dispersion of Cyanin, 41 



•extends to the extreme limits of the spectrum has not yet 

 been determined. Pflliger found no traces of this band, and 

 gives values for the refractive index within its limits. It 

 seemed at first that the reason of this might be found in the 

 difference in the optical properties of fused cyanin and that 

 obtained by the evaporation of an alcoholic solution, but we 

 have found that films prepared in the same way as those 

 used by Pfluger show the band also. 



By dispersing white light into a horizontal spectrum by 

 means of a glass prism, and then dispersing this spectrum 

 vertically by means of a cyanin prism, the dispersion-curve 

 can be shown objectively. This method of crossed prisms, 

 originally due to Newton, and used by Kundt in his study of 

 anomalous dispersion, is decidedly the best for illustrating 

 the matter to students. Unfortunately it cannot be pro- 

 jected on a large scale, but we have succeeded in photographing 

 it by mounting the prisms in front of a telescope lens of 

 about 2 metres focus. By using- the small angle prism the 

 entire spectrum is secured, though the very slight deviation 

 obtained in this case makes the picture unsatisfactory. A 

 number of these curved spectra are shown in Plate III. 



The Michelson interferometer furnishes an easy means of 

 determining the retardation of light in thin films of trans- 

 parent substances, and a somewhat extended study of the 

 dispersion of cyanin has been made with this instrument. It 

 is far easier to obtain thin uniform films of the dye by 

 evaporating the alcoholic solution, than it is to make good 

 prisms in this way, consequently better results were looked 

 for than those obtained by Pfliiger with his prisms. The 

 cyanin was deposited from a hot alcoholic solution on selected 

 plate-glass. To secure uniform films in this way, it is 

 necessary to use absolute alcohol and keep the temperature 

 between 35° and 40°, the operation of coating being per- 

 formed in an air-bath by dipping the plates in the solution 



and setting them on edge to dry. A uniform film was 



© © j 



selected, and one half of the plate was freed from the cyanin, 

 that which remained being bounded by a perfectly straight 

 edge. 



The interferometer was adjusted for horizontal fringes, 

 which are most convenient for measuring small displace- 

 ments, and the plate placed upright before one of the mirrors, 

 with the straight edge bounding the cyanin film perpendicular 

 to the fringes. The central fringe was brought into view 

 by using white light, since it is the only one which can be 

 identified, and furnishes us the only means of telling whether 

 the displacement is half a fringe or a fringe and a half. 



