SECT. TI. LIGHT AND HEAT. 69 



once reflected, are rendered incapable of being again^ 

 reflected at certain angles. For example, a ray in-J 

 cident on a plate of flint glass at an angle of 57 is 

 rendered totally incapable of being reflected at that 

 same angle from another plate of flint glass in a 

 plane at right angles to the first. At the incidence of 

 57 the whole of the ray is polarized : it is the maximum 

 of polarization for flint glass, but there is a partial 

 polarization for every other angle ; the portion of the 

 ray polarized increases gradually up to the maximum, 

 as the incidence approaches to 57. All reflecting 

 surfaces are capable of polarizing light and heat, but 

 the angle of incidence at which the ray is totally polar- 

 ized is different in each substance. Thus, the angle of 

 incidence for the maximum polarization of crown glass 

 is 56 55', and no ray can be totally polarized by reflec- 

 tion from the surface of water unless the angle of 

 incidence is 53 11'. As each substance has its own 

 maximum polarizing angle, the effect is evidently owing 

 to the action of the molecules of matter, and not to any 

 peculiarity in the light or heat. 8 



Light and heat are also polarized by refraction, for 

 certain substances, especially irregularly crystallised 

 minerals like Iceland spar, possess the property of 

 dividing a ray of light or heat passing through them 

 in certain directions into two pencils, namely, the 

 ordinary and extraordinary rays. The first of these is 

 refracted according to the same law as in glass or water, 

 never quitting the plane perpendicular to the refracting 

 surface, while the second does quit that plane, being 

 refracted according to a different and more complicated 

 law. Hence, if a crystal of Iceland spar be held to the 

 eye, two images of the same object will generally be 

 seen of equal brightness. But when they are viewed 



8 ' Connection of the Physical Sciences.' 



