Phase Change hy Reflection 9 



ally transparent films. The experiments on the variation of phase 

 change with thickness of the film would hence lead one to expect 

 poor agreement in the region of the spectrum where the sub- 

 stance is transparent. 



Ftichsin. — The measurements on fuchsin (Table i, Fig. 3) 



TABLE I 

 Fuchsin 



show a decreasing phase change for decreasing wave-length until 

 between 420 and 440 fxjx where coincidence, or at least a minimum 

 phase change, is observed. Beyond this point the phase change 

 suddenly mounts to its highest value, after which a gradual de- 

 crease is observed to 28oyu,ju, where its value is .27 wave-length. 

 The transmission region for this fuchsin film was determined by 

 mounting it before the slit of the spectroscope so that a portion 

 of the light falling on the slit went through the film and the glass 

 plate, while the other passed through the glass plate only. By 

 comparing these two portions in the photograph, it was observed 

 that between z^o and 320 jxfL fuchsin was practically as trans- 

 parent as glass. The abrupt change in phase difference at ^20 nfx. 

 from zero to one half wave-length evidently arises from the re- 

 fractive index of the fuchsin, which is less than glass at 425 iifi, 

 rising above it between 425 and 410 /a/x. 



Doppel-Griin. — The value for the phase change at the surface 

 of this substance goes through two minima in the portion of the 



175 



