ON LIGHT. 357 



ing off at the borders by insensible though rapid degrees. 

 Within this "polarized field/' a vast variety of brilliant 

 and beautiful optical phenomena, hereafter to be de- 

 scribed, are very conveniently and elegantly exhibited. 

 One effect is very striking. If instead of a black velvet 

 backing, the glasses be laid on any bright surface, the 

 printed page of a book, for instance this, which, with- 

 out the interposition of the tourmaline, cannot be dis- 

 cerned for the glare of light reflected from the glasses, 

 becomes distinctly visible, and may be read with facility 

 when that glare is taken off in the manner described. 

 So, too, by looking through a tourmaline plate held trans- 

 versely, on the surface of a pond, at the polarizing angle ; 

 the reflected light from the surface being destroyed, the 

 objects at the bottom, the fishes, &c., are distinctly seen, 

 though completely invisible to a bystander. So, too, by 

 polarizing alternately in a vertical and a horizontal plane, 

 the light of one or more lamps, night signals may be 

 made, and a message transmitted, visible and interpretabk 

 as signals, to a distant spectator provided with a tourma- 

 line plate, while a bystander not so provided, though he 

 see the lamps, will have no suspicion that any such com- 

 munication is in progress.* 



(138.) Polarization of the sky light. The light of a clear 

 and perfectly cloudless blue sky is partially polarized in a 

 plane passing through the sun, the eye, and the point of 

 the sky examined. At each point in that great circle of 



* I mention this to prevent a patent being hereafter taken out 

 "for secret communication at a distance by means of polarized 

 light." 



