460 • LECTURE XXXIX. 



becoming luminous, so that the light of decaying wood, or of two pebbles 

 rubbed together, may be projected precisely M'ith the same velocity, as the light 

 emitted by iron burning in oxygen gas, or by the reservoir of liquid fire on 

 the surface of the sun. Another cause would also naturally interfere with 

 the uniformity of the velocity of light, if it consisted merely in the motion 

 of projected corpuscles of matter; Mr. Laplace has calculated, that if any of 

 the stars were 250 times as great in diameter as the sun, its attraction 

 would be so strong as to destroy the whole momentum of the corpuscles of 

 light proceeding from it, and to render the star invisible at a great distance ; 

 and although there is no reason to imagine that any of thg stars are actually 

 of this magnitude, yet some of them are probably many times greater than 

 our sun, and therefore large enough to produce such a retardation in the 

 motion of their light as would materially alter its effects. It is almost unneces- 

 sary to observe that the uniformity of the velocity of light, in those spaces which 

 are free from all material substances, is a necessary consequence of the Huy- 

 genian hypothesis, since the undulations of every homogeneous elastic 

 medium are always propagated, like those of sound, with the same velocity, 

 as long as the medium remains unaltered. 



On either supposition, there is no difficulty in explaining the equality of 

 the angles of incidence and reflection ; for these angles are equal as well in 

 the collision of common elastic bodies with others incomparably larger, as in 

 . the reflections of the waves of water and of the undulations of sound. And 

 it is equally easy to demonstrate, that the sines of the angles of incidence and 

 refraction must be always in the same proportion at the same surface, 

 whether it be supposed to possess an attractive force, capable of acting on 

 the particles of light, or to be the limit of a medium through which the 

 undulations are propagated with a diminished velocity. There are, however, 

 some cases of the production of colours, which lead us to suppose that the 

 velocity of light must be smaller in a denser than in a rarer medium; and 

 supposing this fact to be fully established, the existence of such an attractive 

 force could no longer be allowed, nor could the system of emanation be 

 maintained by any one. 



The partial reflection from all refracting surfaces is supposed by Newton 

 to arise from certain periodical retardations of the particles of liglit, caused 



