transmitted through crystallized Bodies. 26S 



the biiglit image had reached its greatest brilliancy. The bright 

 and the nebulous images, therefore, comported themselves ex- 

 actly like the two images formed by doubly refracting" crystals; 

 and the small portion of nebulous light, which surrounded the 

 bright image at its maximum lustre, was obviously produced 

 eitlier by the imperfect polish of the agate, or by its not being 

 cut exactly at right angles to the plane of its laminae. 



It will be seen from a subsequent section of this letter, that 

 light "polarized by the agate, or by any other means, is depo- 

 larized, or partly restored to its original state, by being trans- 

 mitted in a particular direction through a plate of mica, or aay 

 other crystallized body. I therefore interposed a plate of mica 

 between the agate and the Iceland spar when the nebulous light 

 had nearly disappeared, and having adjusted it to the depolariz- 

 ing position, the nebulous light was instantly revived round the 

 bright image, while the other bright image which had disap- 

 peared resumed its place in the middle of the other nebulous 

 mass. 



\Vlien a pencil of light polarized and afterwards depolarized, 

 in a manner to be afterwards described, is transmitted through 

 a plate of agate, the ?c^* rays go to the formation of the bright 

 image, while the green rays compose the nebulous light, so that 

 we have a red hright image enveloped in a cloud of green light. 

 Bv turning round the agate 90^ the bright image is formed by 

 the green rays, while the nebulous image consists of the red rays, 

 so that we have a green hright image encircled by a mass of 

 nebulous red light. If in the place of the agate we substitute a 

 doubly refacting crystal, it will always be found that the ordinary 

 image is green when the extraordinary one is red, and that they 

 assume these colours alternately during the motion of the prism 

 round the axis of vision. 



From these experiments, we may consider it as demonstrated, 

 that the nebulous light has the same relation to the bright 

 image, as the first has to the second image of all crystals that 

 have the property of double refraction f. It does not appear, 

 however, that the nebulous image ii produced by a greater re- 

 fractive power than that by which the bright image is formed. 

 There is on the contrary every reason to conclude, in opposition 

 to the analogy of all doubly refracting J crystals, that the agate 



* Tlie red and tlie green are complementary to each other. The same 

 result is obtained if the blue and yellow, or any other two coniplemcntary 

 colours are used. 



t See P^diiiburi;h Trans, vol. vii. part ii. 



I It will be seen from a subsequent paper, that many other bodies both 

 of mineral, animal, and vegetable origin, have the property of forming two 

 imajrcs, polari/ed in an opposite maimer, but not produced by twodiflerent 

 refractive powers. 



R 4 gives 



