STRUCTURE AND LIGHT OF THE SKY. 257 



When a luminous beam impinges at the proper angle 

 on a plane-glass surface it is polarized by reflection. It is 

 polarized, in part, by all oblique reflections ; but at one 

 particular angle, the reflected light is perfectly polarized. 

 An exceedingly beautiful and simple law, discovered by 

 Sir David Brewster, enables us readily to find the polarizing 

 angle of any substance whose refractive index is known. 

 This law was discovered experimentally by Brewster ; but 

 the "Wave Theory of light renders a complete reason for 

 the law. A geometrical image of it is thus given : When 

 a beam of light impinges obliquely upon a plate of glass it 

 is in part reflected and in part refracted. At one particular 

 incidence the reflected and the refracted portions of the 

 beam are at right angles to each other. The angle of inci- 

 dence is then the polarizing angle. It varies with the re- 

 fractive index of the substance ; being for water 52^, for 

 glass 57, and for diamond 68. 



It has been already stated that, in order to obtain the 

 most perfect polarization of the firmamental light, the sky 

 must be regarded in a direction at right angles to the solar 

 beams. This is sometimes expressed by saying that the 

 place of maximum polarization is at an angular distance of 

 90 from the sun. This angle, enclosed as it is between the 

 direct and reflected rays, comprises both the angles of inci- 

 dence and reflection, supposing the polarization to be due 

 to a single reflection. Hence the angle of incidence is half 

 of 90, or 45. This is the atmospheric polarizing angle, 

 and the question is, what known substance possesses an 

 index of refraction to correspond with this polarizing angle ? 

 " If," says Sir John Herschel, " we knew this substance, we 

 might be tempted to conclude that particles of it, scattered 

 in the atmosphere, produce the polarization of the sky. 

 Were the angle of maximum polarization 76 (instead of 

 90), we should look to water or ice, as the reflecting body, 

 however inconceivable the existence in a cloudless atmos- 



