Double Refraction of Quartz. 81 



it appeared that in right-handed quartz it is necessary to sup- 

 pose the right-circular polarization transmitted with the greater 

 velocity; in left-handed quartz the contrary. I have repeated 

 and varied most of Fresnel's experiments relating to this sub- 

 ject, and am perfectly convinced of the correctness of his views. 



Now if, in the experiment with the glass rhomb, the planes 

 of polarization and incidence be inclined at any other angle 

 than 45°, the magnitudes of the undulations parallel and per- 

 pendicular to the plane of incidence will no longer be equal : 

 but the alteration of their periods will be the same as before. 

 The displacement of the particles of ether will still be repre- 

 sented by a helix, but instead of being traced round a circular 

 cylinder, it must be supposed traced round an elliptic cylinder. 

 This modification may properly be called (as Fresnel has called 

 it) elliptical polarization*. This term has since been used by 

 Dr. Brewster to express the nature of the light (probably iden- 

 tical with this, or nearly so) reflected from metallic surfaces. 



ceived on an analyzing plate or tourmaline presents the same appearance in whatever direction 

 the analyzing plate is tnrned round the incident ray. &. The phenomena of depolarization 

 are the same in whatever direction the analyzing plate is turned. 3<. If the polarized light 

 passes through two such rhombs placed in similar positions, the plane of po.arization is 

 shifted 90 o. 4-. If they are placed in crossed portions the plane of polarization is unaltered. 

 ■ The phenomena of depolarization agree with the calculations founded on this supposition • 

 m uniaxal crystals, where the plane of polarization of one ray is changed 360° in going round 

 he axis, the alternate quadrants are pushed in and thrust out one quarter of a tint: and in 

 b.axal crystals, where the plane of polarization is changed only 180" in going round the axis, 

 the alternate semicircles are altered in the same manner. 



* If I might venture to fix on the discovery of Fresnel, which among all his wonderful ad- 

 ditions to opt.cal scence appears likely to possess the greatest practical value, I should select 

 his invention of the mode of producing circularly-polarized or elliptically-polarized light by 

 ■ nternal reflexion of plane-polarized light in glass or water. He has given us the power of 

 producmg light whose laws are as well known as those of plane-polarized light, and which is 

 more manageable, inasmuch as it admits of degrees in its ellipticity. The beautiful geometry 

 ot Malus is forgotten when we think of the discovery of polarization: the far more valuable 

 tneoret.cal discoveries of Fresnel will lose their preeminence when put in competition with an 

 invention which enables others to make discoveries. 

 Vol. IV. Part I. L 



