CONFIRMATION OF THE UNDULATORY THEORY. 489 



the use of polarized light, supplied other striking 

 confirmations of the theory. These were in one 

 case the more remarkable, since the result was 

 foreseen by means of a rigorous application of the 

 conception of the vibratory motion of light, and 

 confirmed by experiment. Professor Airy, of Cam- 

 bridge, was led by his reasonings to see, that if 

 Newton's rings are produced between a lens and a 

 plate of metal by polarized light, then, up to the 

 polarizing angle, the central spot will be black, and 

 instantly beyond this, it will be white. In a note 15 , 

 in which he announced this, he says, "This I anti- 

 cipated from Fresnel's expressions; it is confirma- 

 tory of them, and defies emission." He also pre- 

 dicted that when the rings were produced between 

 two substances of very different refractive powers, 

 the center would twice pass from black to white 

 and from white to black, by increasing the angle; 

 which anticipation was fulfilled by using a diamond 

 for the higher refraction 16 . 



7. Conical Refraction. In the same manner, 

 Professor Hamilton of Dublin pointed out that 

 according to the Fresnelian doctrine of double 

 refraction, there is a certain direction of a crystal 

 in which a single ray of light will be refracted so 



. 15 Addressed to myself, dated May 23, 1831. I ought how- 

 ever to notice, that this experiment had been made by M. Arago, 

 fifteen years earlier, and published : though not then recollected 

 by Mr. Airy. 



^ 6 Camb. Trans, vol. ii. p. 409. 



