4 Oliver H. Gish 



A glass plate having parallel faces should show no interference 

 bands when illuminated with monochromatic light from a broad 

 source; but when white light reflected from it is dispersed by a 

 spectroscope a series of vertical bands extending throughout the 

 spectrum, in ordinary working conditions, may be observed. 

 These bands arise from the fact that waves, of such length that 

 their effective path in the plate is an odd number of half wave- 

 lengths, produce destructive interference in the reflected light. A 

 set of bands from the coated part of the plate and a set from an 

 adjacent uncoated part will be shifted relative to each other. The 

 magnitude of the phase change may be determined from this shift. 



The latter method is essentially that of Wernicke; the former 

 that used by Drude, and by Koenigsberger and Bender. Both 

 have been used in the present work. 



First Method. — Because of the convenience of obtaining thin 

 parallel plates mica was used in part of the work, but other ad- 

 vantages led finally to the use of thin wedge-shaped plates of glass. 

 The substances investigated for phase change were silver, fuchsin, 

 doppel-griin, crystal-violet, cyanin, eosin and aniline-orange. 



First Method 



In the observations with mica the following plan was followed. 

 A mica plate coated, except for a central strip, with the substance 

 to be tested, was so mounted (Fig. i), that a source (s) was 



focused by means of a quartz lens (a) upon the mica (p) at a 

 small angle of incidence (about 2.5 degrees). Then, by means of 

 another quartz lens (&) an image of this plate by the reflected light 



170 



