SECT, xxii.] COLOURED IMAGES. 209 



SECTION XXII. 



Phenomena exhibited by the Passage of Polarized Light through Mica and 

 Sulphate of Lime The Coloured Images produced by Polarized Light pass- 

 ing 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. 



SUCH is the nature of polarized light and of the laws it fol- 

 lows. 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. 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 

 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 re- 

 fracting 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 



