206 



Mr. Horner and Sir David Brewster on 



distinct image becomes brighter and whiter. All the three 

 images, A A, B B, and C C, are 



united into a mass round D, at 



a perpendicular incidence, but W[ 



they separate upon inclining the 



plate, and their distance increases 



with the inclination. 



If we examine the nature of 



the light of which these images 



are composed, we shall find that 



the nebulous images, A A, B B, 



are wholly polarized in a plane 



passing through the direction of 



their length, while C C and the 



greater part of D are polarized 



in an opposite plane. As the 



thickness of the plate increases, 



more and more of the distinct 



image, D, is polarized in the same 



plane, as in mother-of-pearl*, 



till at a certain thickness the 



whole of it is thus polarized. 



In this case all the doubly re- 

 fracted light which forms the ne- 

 bulous image, A A, and the 



bright one, D, consists of two 



oppositely polarized pencils, the one forming the nebulous and 

 the other the distinct image. 



In investigating the cause of these phaenomena, we must take 

 as our guide the analogous facts presented by certain compo- 

 site crystals of calcareous spar. Having long ago described this 

 class of phaenomena very fully f , I shall only state at present 

 the general fact. Let A B C D, fig. 2., be a section of a rhomb 

 of calcareous spar 



having its axis per- Fig. 2. 



pendicular to the faces A, 



A B, C D, and let E F 

 be another crystal, or 

 vein of the same sub- 

 stance crossing it ac- c 

 cording to the law of 

 crystallographic composition. If we now look at a candle 

 through this compound crystal, it will appear single in the di- 

 rection of the axis of A B C D ; but if we incline the plate in 



♦See Phil. Trans., 1814. 



t Ibid., 1815. Edinburgh Encyclopaedia, art. Optics. 



