810 



Professor Sir James Dewar 



[Jan. 21, 



of the transmitted light is reflected on the liquid, and therefore it 

 does not suffer the phase change of half a beat already referred to. 

 The resulting colours Avill therefore be complementary to those 

 produced at the same time in the reflected light, because the amount 

 of interference will be uniformly different by this half beat, the 

 vibrations suppressed in the reflected illumination being present in 

 that transmitted, and vice versa. The limiting case of the very thin 

 film appears bright by the transmitted light, since no appreciable 

 retardation occurs in the passage through the film ; and the phase 

 change of a half beat giving the black film by suppression of the 

 reflected light does not happen in the transmitted light. 



Fig. 15. 



A simple method of obtaining a soap film of about 25 cm. diam. 

 is illustrated in Fig. 15. An inverted cylindrical glass shade A 

 containing slight depths of soap solution has its inside surface 

 thoroughly moistened by careful tilting and rotating. A glass cover 

 pierced with a hole large enough for a convenient length of quill 

 tube B to pass was bent at right angles and connected to the outlet 

 of a gas washing bottle C charged with granular soda lime to purify 

 the air from carbonic gas and cotton wool to remove all suspended 

 matter .^ A bubbler U-tube D attached to the inlet of C serves as a 

 convenient mouthpiece and condenses the major part of the water 



