yo RELATION OF POLARIZATION PART i. 



through a plate of tourmaline it will be found that while 

 the spar remains in the same position the images vary 

 in relative brightness as the tourmaline is made to 

 revolve in the same plane ; one increases in intensity 

 till it arrives at a maximum, at the same time that the 

 other diminishes till it vanishes, and so on alternately 

 at each quarter revolution of the tourmaline, proving 

 both rays to be polarized. For in one position the 

 tourmaline transmits the ordinary ray and reflects the 

 extraordinary, and after revolving 90, the extraordinary 

 ray is transmitted and the ordinary ray is reflected. 



The undulations of the ethereal medium which pro- 

 duce the sensation of common light, are performed in 

 every plane at right angles to the direction in which 

 the ray is moving, but the case is very different after 

 the ray has been polarized by passing through a sub- 

 stance like Iceland spar, for the light then proceeds in 

 two parallel pencils whose undulations are still indeed 

 transverse to the direction of the rays, but they are 

 accomplished in planes at right angles to one another. 

 The ray of common light is like a round rod, whereas 

 the parallel polarized rays resemble two long flat rulers, 

 one of which lies on its broad surface, and the other on 

 its edge. By a simple mechanical law, each vibratory 

 motion of the common light is resolved into vibratory 

 motions at right angles to one another. 



The polarization of light and heat by refraction is not 

 owing to the chemical composition, but to a want of 

 homogeneity in the molecular structure of the substances 

 through which they pass; for regular crystals and 

 substances which are throughout of the same tempera- 

 ture, density, and structure, are incapable of double 

 refraction. The effect of molecular structure is strik- 

 ingly exhibited by the circular polarisation in the 

 dimorphic crystals of quartz. In one form the plane 

 of polarization revolves from right to left, and in the 



