IIISTOEY OF OPTICS. 



desire to avoid as much as possible all reference to theoiy The laws 

 \vh'ck he has obtained, however, belong to elliptically-polarized light 

 in the sense in which the term was introduced by Fresnel." And the 

 identity of the light produced by metallic reflection with the elliptic 

 ally-polarized light of the wave-theory, is placed beyond all doubt, by 

 an observation of Professor Airy, that the rings of nniaxal crystals, 

 produced by Fresnel's elliptically-polarized light, are exactly the same 

 as those produced by Brewster's metallic light. 



6. Newton's Rings by Polarized Light. Other modifications of the 

 phenomena of thin plates by 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 Cambridge, 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 

 anticipated from Fresnel's expressions ; it is confirmatory of them, 

 and defies emission." He also predicted that when the rings were 

 produced between two sdbstances of very different refractive powers, 

 the centre 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. 10 



7. Conical Refraction. In the same manner, Professor Hamilton 

 of Dublin pointed out Hi at 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 as to form a conical pencil. 

 For the direction of the refracted ray is determined by a plane which 

 touches the wave surface, the rule being that the ray must pass from 

 the centre of the surface to the point of contact ; and though in 

 general this contact gives a single point only, it so happens, from the 

 peculiar inflected form of the wave surface, which has what is called 

 # cus}}, that in one particular position, the plane can touch the surface 

 in an entire circle. Thus the general rule which assigns the path of 



15 Addressed to myself, dated May 23, 1831. I ought, however, to notice, 

 that this experiment had been made by M. Arago, fifteen years earlier, and 

 published : though not then recollected by Mr. Airy. 



16 Camb. Trans, vol. ii. p. 409. 



