186 COLOUEED IMAGES. SECT. XXII. 



SECTION XXII. 



Phenomena exhibited by the Passage of Polarized Light through Mica and 

 Sulphate of Lime The Coloured 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 Coloured Images produced by the Interference of 

 Polarized Rays Fluorescence. 



SUCH is the nature of polarized light and of the laws it follows. 

 But it is hardly possible to convey an idea of the splendour of the 

 phenomena it exhibits under circumstances 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 longitudinal section 

 vertical, an obscure cloud, with its centre totally^ dark, will be 

 seen on the glass. Now, let a plate of mica, uniformly about the 

 thirtieth of an inch in thickness, be interposed between the 

 tourmaline and the glass ; the dark spot will instantly vanish, 

 and, instead of it, a succession of the most gorgeous colours will 

 appear, varying with every inclination of the mica, from the 

 richest reds, to the most vivid greens, blues, and purples (N. 

 211). 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 perpendicular to the polarized ray, it 

 will be found that there are two lines in it where the colours 

 entirely vanish. These are the optic axes of the mica, which is 

 a doubly refracting substance, with two optic axes, along which 

 light is refracted in one pencil. 



No colours 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 coloured light complementary to 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 sulphate of lime, between the twentieth and sixtieth of 



