BREATHING LIGHT AND DARKNESS. 197 



spectator at E. upon looking into the two mirrors C, D f 

 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 A C, B D, is polarized by reflection 

 from the plates A, B, because it is incident at the polariz- 

 ing 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 A C, B D, fell upon the plates C, D, at 

 an angle of 53 11', the candle from which they proceeded 

 would not be visible, or they would not suffer reflection 

 from the plates C D. At all other angles the light would 

 be reflected and the candles visible. Now the plate D 

 is placed at an angle of 53 11', and C at an angle of 

 56 45', so that when a film of water is breathed upon 

 them the light will be reflected from the latter, and none 

 from the former : that is, the act of breathing upon the 

 plates will restore the invisible, and extinguish the visible 

 image. 



