MAGNUSSON — ANOMALOUS DISPERSION OF CYANIN. 



277 



To show the similarity between this effect and the action due 

 to the absorption in the green, figures 5 and 6 of plate 19 were 

 taken. In the former the time of exposure for the direct ray- 

 was forty-five seconds and for the cyanin five minutes. This 

 gives approximately the same intensity for the lines of shorter 

 wave length in the photograph, but for the longer wave no im- 

 pression is seen in the lower band. Figure 6 is the same as 

 figure 5 save the time of exposure, which in this case was forty 

 seconds for both sets. By comparing the wave lengths with 

 the absorption curve it is seen that the photographs of figures 

 5 and 6 are at the edge of the absorption band in the yellow. 



Fig. 6. 



'(c) The Method of Crossed Prisms. — In Newton's method 

 of crossed prisms we have a beautiful means of showing the phe- 

 nomenon of anomalous dispersion, for by it we can bend a 

 straight, continuous spectrum into a curve identical in every re- 

 spect with the dispersion curve obtained by measurements with 

 the spectrometer. The method was used by Kundt in the course 

 of his investigations on anomalous dispersion. This method 

 furnishes qualitative data only, but the fact that the curve of 

 dispersion is directly photographed gives value to the results. 

 The general arrangement of the apparatus is seen in figure 6. 



