42 . Prof. Wood and Mr. Magnusson 



& 



on 



The interferometer was then illuminated with monochromatic 

 light from the large direct-vision spectroscope, and the 

 double-fringe system photographed for various wave-lengths. 

 " Er) thro ? plates were used, enabling us to secure records 

 from the extreme red to the ultra-violet, a series of about 

 thirty exposures being made on a single plate. These photo- 

 graphs show the continuity of the dispersion-curve through 

 the absorption-band most beautifully. Beginning in the red 

 we find a large displacement, which increases progressively 

 as we near the region of the absorption-band in the yellow. 

 As soon as we are in this portion of the spectrum, the dis- 

 placement rapidly grows less, the fringes getting almost into 

 line in the middle of the yellow. Then the displacement 

 begins to increase again as we leave the absorption-band on 

 the blue side, the rate of increase growing less as we near 

 the ultra-violet. 



The displacements were measured on the plates by means 

 of a filar micrometer, and were recorded in terms of fringe 

 width. Since the light passes through the film twice, the 

 total displacement is that which corresponds to a retardation 

 of a fiim of twice the thickness. To calculate the refractive 

 index from the retardation we require the thickness of the 

 film. In order to avoid the many sources of error involved 

 in any attempt to measure the film's thickness directly, we 

 made use of the values already determined for the refractive 

 index for those wave-lengths most freely transmitted by the 

 cyanin. The prism method is the more accurate when reason- 

 ably large angles can be used, and the investigations with 

 the interferometer were made with a view of corroborating 

 our results in the absorption-band. Professor Ames has 

 drawn our attention to the fact that in using a prism of a 

 strongly absorbing medium the amplitude of the transmitted 

 wave falls off very rapidly from the refracting edge, and that 

 this might have some effect on the propagation of the wave, 

 for in treating wave propagation we always assume the 

 amplitude to be the same at every point on the wave front. 

 This is a most pertinent suggestion, for if a rapid decrease 

 of amplitude on a wave front changes the position of the 

 effective point to which we can reduce the whole wave (by 

 Huygens's principle) it should certainly modify to some 

 degree the apparent refractive index as determined by a prism.. 

 So far as known no one has ever worked out Huygens's 

 principle for a wave of variable amplitude. There seems to 

 be no way of making the usual geometrical treatment cover 

 the case, although it seems at first sight as if the method of 

 strip-division might be used. 



