SECT. XXII. COLOURED IMAGES. 187 



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 maxi- 

 mum when the tourmaline has turned through ninety degrees ; 

 after that, the green will vanish and the red will reappear, 

 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 Iceland 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 becoming 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 perpen- 

 dicular 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 equiva- 

 lent to a variation in thickness. 



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

 such an inclination as to transmit the polarized ray along one of 

 its optic axes, is viewed through the tourmaline with its axis 

 vertical, a most splendid appearance is presented. The cloudy 

 spot in the direction of the optic axis is seen surrounded by a set 

 of vividly coloured rings of an oval form, divided into two 

 unequal parts by a black curved band passing through the cloudy 

 spot about which the rings are formed. The other optic axis of 

 the mica exhibits a similar image (N. 212). 



When the two optic axes of a crystal make a small angle with 

 one another, as in nitre, the two sets of rings touch externally ; 



