Phase Change by Reflection 7 



division. However this should in no case have caused a greater 

 error than one per cent. The accuracy with which a setting on 

 the center of a band could be repeated depended largely upon the 

 intensity of the photograph. 



Method of Depositing Films 



The silver films used were deposited by Brashear's method and 

 were of such density that only a trace of light could be seen on 

 looking through at the bright sky. 



In obtaining films of the dyes, these were first dissolved in abso- 

 lute alcohol and as concentrated a solution was used as could be 

 dried on the plate without the substance crystallizing. The plates 

 were coated by dipping them in the solution and quickly drying 

 over a Bunsen flame. Considerable difhculty was met in obtaining 

 coatings of heavy and uniform thickness. This lack of uniformity 

 in the coatings is the cause of some irregularities observed in the 

 measurements. In the region of the spectrum where the substance 

 is most transparent distortions of the bands probably arising from 

 this cause could usually be observed. 



Thickness of Films 



Some measurements of the thickness of the films used were made 

 as follows : Interference bands formed by laying another glass 

 plate on the coated side of the plate holding the film to be meas- 

 ured showed a shift between the bands over the coated portion 

 and those over the cleared portion of the plate. This shift was 

 measured for two wave-lengths far enough apart so that a meas- 

 ureable difference in path in wave-length could be observed. The 

 shift of the bands due to the thickness of the coating was meas- 

 ured in the direction in which their gain over those from the 

 coating surface took place, as light of shorter wave-length was 

 used. By varying the wave-lengths of the light illuminating the 

 plate it was readily seen that the thickness was in all measurements 

 less than a wave-length, provided the phase change be neglected. 

 Since fuchsin is very transparent in the red its phase change there 

 should be the same approximately as that of glass, provided its 

 refractive index is greater than one. Under this assumption, that 



173 



