POLARIZATION. 429 



drawings, so as to produce images upon the retinas of the 

 two eyes like those produced by the 

 solid object, we obtain the idea of 

 solidity. 



666. The Stereoscope. To 



blend these two pictures is the office 

 of the stereoscope. Its action will 

 be readily understood from Fig. 336. 

 The diaphragm D prevents either eye 

 from seeing both pictures at the same 

 time. Kays of light from B are re- 

 fracted by the half-lens E' so that they 



seem to come from C. In the same 



FIG. 336. 

 way, rays from A are refracted by E so 



that they also seem to come from C. The two slightly 

 different pictures thus seeming to be in the same 

 place at the same time are successfully blended, and 

 the picture "stands out," or has the appearance of 

 solidity. If the two pictures of a stereoscopic view were 

 exactly alike, this impression of solidity would not be pro- 

 duced. 



667. Polarization. If a horizontal string, tightly 

 drawn, be hit a vertical blow, a wave will be formed with 

 vibrations in a vertical plane. If the string be hit a 

 horizontal blow, a wave will be formed with vibrations in 

 a horizontal plane. Thus a transversal wave is capable of 

 assuming a particular side or direction while a longitudinal 

 wave is not. This is expressed by saying that a transversal 

 wave is capable of polarization. Polarization of light 

 may be produced in three ways by absorption, by reflec- 

 tion and by double refraction. 



