SKCT. XXII. COLORED IMAGES. 181 



one another, that is, which taken together would make 

 white light. One 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 sul- 

 phate of lime between the twentieth and sixtieth of an 

 inch thick be used instead of the mica. When the film 

 is of uniform thickness, only one color will be seen when 

 it is placed between the analyzing plate and the reflect- 

 ing glass ; as, for example, red. But when the tourma- 

 line revolves, the red will vanish by degrees till the film 

 is colorless ; then it will assume a green hue, which 

 will increase and arrive at its maximum when the tour- 

 maline has turned through ninety degrees ; after that 

 the green will vanish and the red will reappear, alter- 

 nating at each quadrant. Thus the tourmaline separ- 

 ates 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 colors every quarter 

 revolution of the spar, the red becoming green, and the 

 green red^ and where the images overlap, the color 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 what- 

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

 pendicular to the polarized ray ; but films of interme- 

 diate thickness will give all colors. 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 colors when polarized light is trans- 

 mitted 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 eyes as 

 possible at such an inclination as to transmit the polar- 

 ized ray along one of its optic axes, is viewed through the 

 tourmaline with, its axis vertical, a most splendid appear- 



Q 



