LIGHT. 89 



ing obliquely out of one medium into another, it undergoes a 

 change of direction. If the second medium be denser than the 

 first, the ray of light is bent, or refracted., nearer to the perpen- 

 dicular ; but in passing out from a denser into a rarer medium, 

 it is refracted from the perpendicular. Thus, when the ray of 

 r\ light r, passing through the air, 



X^^ falls obliquely upon a plate of 



glass at the point a, instead of 

 continuing to move in the same 



^sX^ straight line a b y it is bent to- 



wards the perpendicular at a, 



and proceeds in the direction a c. The ray is bent to the side 

 on which there is the greatest mass of glass. On passing out 

 from the glass into the air, a rarer medium, at the point c, the 

 ray has its direction again changed, and in this case from the 

 perpendicular, but stiU towards the mass of glass. The amount 

 of refraction, generally speaking, is proportional to the density 

 of a body, but combustible bodies possess a higher refracting 

 power than corresponds to their density. Hence the diamond, 

 melted phosphorus, naphtha, and hydrogen gas, exhibit this 

 effect upon light in a greater degree than other transparent 

 bodies. Dr. WoUaston had recourse to this refracting power 

 as a test of the purity of some substances. Thus, genuine oil of 

 cloves had a refracting power expressed by the number 1535, 

 while that of an impure specimen was not more than 1498. 



4. In passing through many crystallized bodies, such as Ice- 

 land spar, a certain portion of light is refracted in the usual 

 way, and another portion undergoes an extraordinary refraction, 

 in a plane parallel to the diagonal which joins the two obtuse 

 angles of the crystal. Such bodies are said to refract doubly, 

 and exhibit a double image of any body viewed through them. 



5. Reflected and likewise doubly refracted light assume new 

 properties. Common light, reflected from the surface of glass, 

 or any bright surface non-metallic, is, more or less of it, con- 

 verted into what is called polarized light. If it be reflected at 

 one particular angle of incidence, 56 45', it is all changed into 

 polarized light ; and the farther the angle of reflection deviates 

 from 56, on either side, the less is polarized, and the more 

 remains common light. 56 is the maximum polarizing angle 

 for glass ; 52.45' for water. The light is said to be polarized, 

 from certain properties which it assumes, which seem to indicate 



