28 Theory of the Aether in the Seventeenth Century. 



experimenting with the Iceland crystal. He observed that the 

 two rays which are obtained by the double refraction of a single 

 ray afterwards behave in a way different from ordinary light 

 which has not experienced double refraction ; and in particular, 

 if one of these rays is incident on a second crystal of Iceland 

 spar, it gives rise in some circumstances to two, and in others 

 to only one, refracted ray. The behaviour of the ray at this 

 second refraction can be altered by simply rotating the second 

 crystal about the direction of the ray as axis ; the ray under- 

 going the ordinary or extraordinary refraction according as the 

 principal section of the crystal is in a certain direction or in the 

 direction at right angles to this. 



The first stage in the explanation of Huygens' observation 

 was reached by Newton, who in 1717 showed* that a ray 

 obtained by double refraction differs from a ray of ordinary 

 light in the same way that a long rod whose cross-section is a 

 rectangle differs from a long rod whose cross-section is a circle : 

 in other words, the properties of a ray of ordinary light are the 

 same with respect to all directions at right angles to its direction 

 of propagation, whereas a ray obtained by double refraction 

 must be supposed to have sides, or properties related to special 

 directions at right angles to its own direction. The refraction 

 of such a ray at the surface of a crystal depends on the relation 

 of its sides to the principal plane of the crystal. 



That a ray of light should possess such properties seemed to 

 Newton f an insuperable objection to the hypothesis which 

 regarded waves of light as analogous to waves of sound. On 

 this point he was in the right : his objections are perfectly 

 valid against the wave-theory as it was understood by his 

 contemporaries J, although not against the theory which was put 

 forward a century later by Young and Fresnel. 



* The second edition of Newton's Opticks, Query 26. t Opticks, Query 28. 



J In which the oscillations are performed in the direction in which the wave 

 advances. 



In which the oscillations are performed in a direction at right angles to that 

 in which the wave advances. 



