24 



SIX LECTURES ON LIGHT. 



We have now to exhibit the bearings of 

 crystallization upon optical phenomena. Ac- 

 cording to the undulatory theory, the velocity 

 of light in water and glass is less than in 

 air. Consider, then, a small portion of a 

 wave issuing from a point of light so distant 

 that the portion may be regarded as practi- 

 cally straight. Moving vertically downwards, 

 and impinging on an horizontal surface of 

 glass, the wave would go through the glass 

 without change of direction. But, as the 

 velocity in glass is less than the velocity in 

 air, the wave would be retarded on passing 

 into the denser medium. 



But suppose the wave, before reaching the 

 glass, to be oblique to the surface ; that end 

 of the wave which first reaches the glass will 

 be the first retarded, the other portions as 

 they enter the glass being retarded in succes- 

 sion. This retardation of the one end of the 

 wave causes it to swing round and change its 

 front, so that when the wave has fully entered 

 the glass its course is oblique to its original di- 

 rection. According to the undulatory theory, 

 light is thus refracted. 



In water, fcr example, there is nothing in 

 the grouping of the molecules to interfere 

 with the perfect homogeneity of the ether ; 

 but, when water crystallizes to ice, the case 

 is different. In a plate of ice the elasticity 

 of the ether in a direction perpendicular to 

 the surface of freezing is different from what 

 it is parallel to the surface of freezing ; ice is. 

 therefore, a double refracting substance. 

 Double refraction is displayed in a particu- 

 larly impressive manner by Iceland spar, 

 which is crystallized carbonate of lime. The 

 difference of ethereal density in two direc- 

 tions in this crystal is very great, the separa- 

 tion oi the beam into the two halves being, 

 therefore, particularly striking. 



Before you is now projected an image of our 

 carbon-points. Introducing the spar, the beam 

 which builds the image is permitted to pass 

 through it; instantly you have the single image 

 divided into two. Projecting an image ot the 

 aperature through which the light issues from 

 the electric lamp, and introducing the spar, 

 two luminous ti k;, insteadof one, appear 

 immediately upon the screen. (See Fig. Q.X 



FIG. 9. 



The two elements of rapidity of propaga- 

 tion, both of sound and light, in any sub- 

 stance whatever, are elasticity and density, 

 and the enormous velocity of light is attain- 

 able because the ether is at the same time of 

 infinitesimal density and of enormous elas- 

 ticity. It surrounds the atoms of all bodies, 

 but seems to be so acted upon by them that 

 its density is increased without a proportionate 

 increase of elasticity ; this would account for 

 the diminished velocity of light in refracting 

 bodies. In virtue of the crystalline archi- 

 ' lecture that we have been considering, the 

 ether in many crystals possesses different j 

 densities in different directions ; and the con- 

 sequence is, that some of these media trans- 

 mit light with two different velocities. Now, 

 refraction depends wholly upon the change 

 of velocity on entering the refracting medium ; 

 and is greatest where the change of volicity 

 is greatest. Hence, as, in many crystals, we 

 have two different velocities, we have also 

 two different refractions, a beam of light being 

 divided by such crystals into two. This ef- 

 fect Uattad<tifai& refraction. 



The two beams into which the spar divides 

 the single incident-beam do not behave alike. 

 One of them obeys the ordinary law of re- 

 fraction discovered by Snell, and this is 

 called the ordinary ray. The other does not 

 obey the ordinary law. Its index of refrac- 

 tion, for example, is not constant, nor do the 

 incident and refracted rays always lie in the 

 same plane. It is, therefore, called the ex- 

 traordinary ray. Pour water, and bisulphide 

 of carbon into two cups of the same depth ; 

 looked at through the liquid, the cup that con- 

 tains the more strongly-refracting liquid will 

 appear shallower than the other. Place a 

 piece cf Iceland spar over a dot of ink ; two 

 dots are seen, but one appears nearer than the 

 other. The nearest dot belongs to the most, 

 strongly-refracted ray, which in this case is 

 ths ordinary ray. Turn the spar round, and 

 the extraordinary image of the spot rotates 

 roi:nd the ordinary one. 



The double refraction of Iceland spar was 

 first treated in a work published by Erasmus 

 Bartholimus, in 1669. The celebrated Huy- 

 ghens sought to account for the phenomenon 



