REPORT ON PHYSICAL OPTICS. S79 



of similar forces emanating from the molecules of the crystal ; 

 but modified by the form of these molecules and those of light, 

 and by the manner in which they are presented to each other. 

 No attempt, however, has been made in the theory of emission to 

 advance beyond the point to which Newton arrived, and to de- 

 duce the velocity of the extraordinary ray in crystallized media 

 from any assumed constitution of the molecular forces* ; and, 

 indeed, when the condition of polarity is to be superadded to 

 the laws of such forces, the theory seems embarrassed in 

 inextricable difficulties. The refraction which a polarized ray 

 undergoes in a crystal depends upon its plane of polarization, 

 and, by a simple change of that plane, the refracted ray may be 

 converted from an extraordinary to an ordinary ray. The extra- 

 ordinary force then, it appears from the phenomena, exerts no 

 effect upon a ray polarized parallel to the principal plane ; its 

 effect is greatest upon a ray polarized in the pei-pendicular plane ; 

 and it must be supposed to act in every intermediate degree 

 upon rays polarized in intermediate planes. Now a ray of com- 

 mon light, in the theory of emission, is composed of molecules 

 whose planes of polarization are turned in all azimuths; and 

 these molecules, consequently, should feel the influence of the 

 extraordinary force in every possible degree. Instead, therefore, 

 of two refracted rays, such a ray should be divided into an infinite 

 number, inclined in every possible angle between the limiting 

 directions of the ordinary and extraordinary rays. 



It had been hitherto assumed, that no crystal had more than 

 one optic axis. While examining the rings which surround 

 these axes in polarized light. Sir David Brewster made the im- 

 portant discovery that the greater number of crystals possess 

 ttvo optic axes ; and he soon after discovered the connexion be- 

 tween these diversities of optical character and the crystalline 

 formf. 



The optic axes, however, as Sir David Brewster has shown, 

 cannot be regarded in general as the fundamental axes of the 

 double-refracting medium. He calls them apparent axes; and 

 considers them as the resultants of others, which he denominates 



* Fresnel states, in the commencement of his memoir on double refraction, 

 that Laplace had derived the velocity of the extraordinary ray, in uniaxal cry- 

 stals, from the hypothesis of a resultant force acting in a direction pei-pendicular 

 to the optic axis, and varying as the square of the sine of the angle which the 

 ray makes with that line. I have not been able to discover, in any of La- 

 place's writings, the discussion thus adverted to. 



\ The important relations here alluded to have been already brought under 

 the attention of the Association, in the able Report on Miucralocv, bv Mr. Whe • 

 well. ^" " 



