210 COLOUKED IMAGES. [SECT. xxn. 



of these it absorbs, and transmits the other ; it is, therefore, 

 called the analyzing plate. The truth of this will appear 

 more readily if a film of sulphate of lime, between the twen- 

 tieth and sixtieth of an inch thick, be used instead of the 

 mica. When the film is of uniform thickness, only one colour 

 will be seen when it is placed between the analyzing plate 

 and the reflecting glass ; as, for example, red. But, when 

 the tourmaline revolves, the red will vanish by degrees till 

 the film is colourless; then it will assume a green hue, 

 which will increase and arrive at its maximum when the 

 tourmaline has turned through ninety degrees ; after that, 

 the green will vanish and the red will re- appear, alternating 

 at each quadrant. Thus the tourmaline separates the light 

 which has passed through the film into a red and a green 

 pencil; in one position it absorbs the green and lets the 

 red pass, and in another it absorbs the red and transmits 

 the green. This is proved by analyzing the ray with Ice- 

 land spar instead of tourmaline ; for, since the spar does not 

 absorb the light, two images of the sulphate of lime will be 

 seen, one red and the other green ; and these exchange 

 colours every quarter revolution of the spar, the red be- 

 coming green, and the green red ; and, where the images 

 overlap, the colour is white, proving the red and green to 

 be complementary to each other. The tint depends on the 

 thickness of the film. Films of sulphate of lime, the 0-00124 

 and 0-01818 of an inch respectively, give white light in 

 whatever position they may be held, provided they be per- 

 pendicular to the polarized ray ; but films of intermediate 

 thickness will give all colours. Consequently, a wedge of 

 sulphate of lime, varying in thickness between the 0-00124 

 and the 0-01818 of an inch, will appear to be striped with 

 all colours when polarized light is transmitted through it. 

 A change in the inclination of the film, whether of mica or 

 sulphate of lime, is evidently equivalent to a variation in 

 thickness. 



When a plate of mica, held as close to the eye as possible, 



