

(506.) 

 Observa- 

 tion of 

 JIalus, 

 Arago, Sir 

 T). Brew- 

 ster, and 

 M. Biot, 



CHAP. V., 4.] 



OPTICS. ARAGO. 



Ill 





attributed 

 to inter- 

 ference by 

 Younsr. 



Case of 

 rays oppo- 

 sitely po- 

 larized 

 Arago and 

 Fresnel. 



(507.) 

 Depolari- 

 zation ex- 

 plained on 

 the undu- 

 latory 

 theory. 



the positions in which the light passed through un- 

 changed. 



The theory of this simple yet admirable experi- 

 ment is one of the happiest examples of Fresnel's 

 mechanical explanation of double refraction. But 

 it was not attained by a single step, nor effected 

 by a single hand. Malus had observed the fact of 

 depolarization, and the existence of perpendicular 

 neutral axes in the crystalline plate. Arago added 

 to this the knowledge of the phenomena of colour, 

 which Sir David Brewster also observed indepen- 

 dently somewhat later. He also invented a par- 

 ticular theory to explain them by what he termed 

 move-able polarization. But it was not a success- 

 ful effort. M. Biot assiduously studied the empi- 

 rical laws of these periodic colours, and traced 

 their dependence on the thickness of the interposed 

 plate, according to a law similar to that of Newton's 

 rings. Young made an important step farther. He 

 attributed the colour to the interference of the ordi- 

 nary and extraordinary rays into which the incident 

 light was divided on entering the crystalline plate ; 

 and he showed by accurate calculation that the retar- 

 dation of the slower- moving of the two rays during 

 their passage through the plate, did in fact produce 

 the difference of phase necessary for developing the 

 tints observed by their reunion on leaving the plate, 

 according to the usual laws of interference ; and he 

 showed that this theory coincided with M. Biot's 

 rules. 1 But even this was not enough to explain the 

 facts. It was not clear why the colours due to doubly- 

 refracting plates should not be seen without reaching 

 the eye through an analyzer, or calc-spar prism. To 

 Arago and Fresnel jointly we owe the important 

 reply to this difficulty, which in fact forced upon the 

 latter the idea of transversal vibrations. Their joint 

 experiments (mentioned in the last section, art. 488), 

 had shown that oppositely polarized rays cannot in- 

 terfere unless they have, first, a common origin and a 

 common plane of polarization ; and, secondly, unless 

 they be reduced to a common plane of polarization 

 (that is, analyzed} before falling on the eye or the 

 screen : so that the theory of the colours of crystal- 

 lized plates is, briefly, as follows : 



Polarized light is represented by transverse vibra- 

 tions of ether, the particles moving all in one plane. 

 The crystallized plate has, we will suppose (in order 

 to take the simplest case) one axis of double refrac- 

 tion, and the dii'ection of that axis is in the plane of 

 the lamina. When the vibrations of the light falling 

 on the crystallized plate are either wholly parallel or 

 wholly perpendicular to this axis, the light is trans- 

 mitted without alteration, either as an extraordinary 

 or as an ordinary ray, and it is then reflected or not 

 by the analyzer, as would have been the case had it 

 not been transmitted at all through the crystal of 



selenite. But if the axis of the crystal be inclined, 

 suppose 45, to the plane of vibration of the incident 

 polarized ray. the vibration is mechanically resolved 

 into two, which are oppositely polarized, After tra- 

 versing the thickness of the crystallized plate with 

 the different velocities due to the motion of the ordi- 

 nary and extraordinary rays, they are reunited at 

 emergence, but in altered relative phases, so that by 

 their union they form a beam no longer polarized 

 (unless by an exception) in the same plane as at first, 

 nor indeed plane-polarized at all, but most likely per- 

 forming elliptical or circular vibrations, which, again, 

 falling on the analyzer, are reflected or transmitted, 

 or partly both, in a manner quite different from what 

 would have happened to the light unchanged by crys- 

 talline transmission. 



White light becomes coloured because the state of ( 508 -) 

 polarization of the emergent ray depends on the differ- thTcolour. 

 ence of length of path for the two rays which under- 

 went crystalline separation within the plate, and also 

 on the length of a wave of light (for this determines the 

 phase of polarization at emergence). But the wave 

 of light varies in length for each colour, consequently 

 every colour has its maximum under different circum- 

 stances, and if the incident light be white, the light 

 falling after reflection on a screen will present mixed 

 tints similar to those of Newton's rings. 



This extraordinary property of a crystallized plate (509.) 

 (which, in common light, appears equally transparent Similar 

 and homogeneous in every direction), of modifying: P hen0 ' . 



,. i , -P . ., .,, .i- 7 i menon m 



light, or dyeing it with the most gorgeous colours, radiant 

 when the plate is merely turned in its own plane, is heat. 

 one of the nicest tests of the polarization of the light, 

 and has been used to detect the analogous polariza- 

 tion of radiant heat, and the concomitant phenome- 

 non of double refraction, which, except in the case 

 of the heat accompanying the solar rays, has not 

 yet been independently recognized. 2 



Arago applied his discovery to the construe- (510.) 

 tion of a polariscope, for estimating the feeblest ^8 s i )0 ' 



/.-,.. T , lariscope. 



amount or polarization ; and he used this instru- 

 ment for some very interesting experiments on the 

 polarization of the light of the sky (which is sun- 

 light polarized by reflection from the atmosphere), 

 and on that of different incandescent and reflecting 

 surfaces. He also found that the moon and the 

 tails of comets send light to the eye which is slightly 

 polarized, thus betraying its borrowed origin. But 

 that of the sun, being absolutely neutral, is only 

 comparable (according to Arago) to the light arising 

 from incandescent vapours, thus distinguishing the 

 sun from a solid or liquid globe. 



We cannot do more than allude to Arago's other 

 optical papers and experiments. He was, probably, 

 the only Frenchman of his time who was well ac- 

 quainted with Young's discoveries. The explana- rings and 



(511.) 



1 Quarterly Review, vol. xi. ; Miscell. Works, vol. i., p. 266. 

 2 See Researches on Ileat by the present writer. Edinburgh Transactions, vols. xiii. and xiv. 



