BREATHING LIGHT AND DARKNESS. 



125 



eye placed at E, and looking at the same time 

 into the two plates C and D, will see very faint 

 images of the candle S, which by a slight adjust- 

 ment of the plates, may be made to disappear 

 Fig. 29. 



almost wholly allowing the plate C to remain as 

 it is, change the position of D, till its inclination 

 to the ray BD is diminished about 3, or made 

 nearly 53 11'. When this is done, the image 

 that had disappeared on looking into D will be 

 restored, so that the spectator at E, upon looking 

 into the two mirrors C,D, will see no light in C, 

 because the candle has nearly disappeared, while 

 the candle is distinctly seen in D. If, while the 

 spectator is looking into these two mirrors, either 

 he or another person breathes upon them gently 

 and quickly, the breath will revive the ex- 

 tinguished image in C, and will extinguish the 

 visible image in D. The following is the cause 

 of this singular result. The light AC, BD, is 

 polarized by reflexion from the plates A, B, 

 because it is incident at the polarizing angle of 

 56 45' for glass. When we breathe upon the 

 plates C, D, we form upon their surface a thin 

 film of water, whose polarizing angle is 53 11', 

 so that if the polarized rays AC, BD, fell upon 

 the plates C, D, at an angle of 53 11', the 



