180 COLORED IMAGES. SfccT. XXII. 



greater or less than a polarizing angle, a part only of 

 the reflected ray will be polarized, but a part of what is 

 transmitted will be polarized by reflection at the sur- 

 face of the second plate, part at the third, and so on till 

 the whole is poralized. This is the best apparatus ; but 

 one plate of glass having its inferior surface blackened, 

 or even a polished table, will answer the purpose. 



''" ' 



SECTION XXII. 



Phenomena exhibited by the passage of Polarized Light through Mica and 

 Sulphate of Lime The Colored Images produced by Polarized Light 

 passing through Crystals having one and two Optic Axes Circular 

 Polarization Elliptical Polarization Discoveries of MM. Biot, Fresnel, 

 and Professor Airy Colored Images produced by the Interference of 

 Polarized Rays. 



SUCH is the nature of polarized light and of the laws 

 it follows. But it is hardly possible to convey an idea of 

 the splendor of the phenomena it exhibits under circum- 

 stances which an attempt will now be made to describe. 



If light polarized by reflection from a pane of glass be 

 viewed through a plate of tourmaline, with its longitudi- 

 nal section vertical, an obscure cloud, with its center 

 totally dark, will be seen on the glass. Now let a plate 

 of mica, uniformly about the thirtieth of an inch in thick- 

 ness, be interposed between the tourmaline and the 

 glass ; the dark spot will instantly vanish, and instead of 

 it, a succession of the most gorgeous colors will appear, 

 varying with every inclination of the mica, from the 

 richest reds, to the most vivid greens, blues, and purples 

 (N. 206). That they may be seen in perfection, the 

 mica must revolve at right angles to its own plane. 

 When the mica is turned round in a plane perpendicu- 

 lar to the polarized ray, it will be found that there are 

 two lines in it where the colors entirely vanish. These 

 are the optic axes of the mica, which is a doubly refract- 

 ing substance, with two optic axes, along which light is 

 refracted in one pencil. 



No colors are visible in the mica, whatever its position 

 may be with regard to the polarized light, without the 

 aid of the tourmaline, which separates the transmitted 

 ray into two pencils of colored light complementary to 



