REFLEXION AND REFRACTION. 21 



(24) The intensity of the reflected light, on the other hand, 

 is found to vary greatly with the medium. The following lead- 

 ing facts have been established experimentally. 



I. The quantity of light regularly reflected increases with 

 the angle of incidence, the increase being very slow at mo- 

 derate incidences, and becoming very rapid at great ones. 

 Thus, water at a perpendicular incidence, according to the 

 experiments of Bouguer, reflects only 18 rays out of 1000 ; 

 at an incidence of 40 it reflects 22 rays ; at 60, 65 rays ; 

 at 80, 333 rays ; and at 89|, 721 rays. 



II. The quantity of light reflected at the same incidence 

 varies both with the medium upon which the light falls, and 

 with that from which it is incident. Thus, at a perpendicu- 

 lar incidence, the number of rays reflected by water, glass, 

 and mercury, are 18, 25, and 666, respectively, the number 

 of incident rays being 1000. The dependence of the quantity 

 of the reflected light upon, the medium from which it is in- 

 cident is easily shown by immersing a plate of glass in water 

 or oil. 



III. The differences in the reflective powers of different 

 substances are much more marked at small, than at great 

 incidences. Thus, water and mercury the first of which 

 reflects but the one-fiftieth part of the incident light at a 

 perpendicular incidence, while the latter reflects two-thirds 

 are equally reflective at an incidence of 89^, the number 

 of rays reflected at this angle being, in both cases, 721 out of 

 1000. 



(25) When light is incident upon the surface of a trans- 

 parent medium, a portion enters the medium, pursuing there 

 an altered direction. This portion is said to be refracted. 



When the ray passes from a rarer into a denser medium, 

 the angle of incidence is, in general, greater than the angle of 

 refraction, and the deviation takes place towards the perpendi- 



