362 LECTURE XXXIX. 



The partial reflection from all refracting surfaces is supposed by Newton 

 to arise from certain periodical retardations of the particles of light, 

 caused by undulations, propagated in all cases through an ethereal me- 

 dium. The mechanism of these supposed undulations is so complicated, 

 and attended by so many difficulties, that the few who have examined 

 them have been in general entirely dissatisfied with them ; and the internal 

 vibrations of the particles of light themselves, which Boscovich has 

 imagined, appear scarcely to require a serious discussion. It may, there- 

 fore, safely be asserted, that in the projectile hypothesis this separation of 

 the rays of light of the same kind by a partial reflection at every refract- 

 ing surface, remains wholly unexplained. In the undulatory system, on 

 the contrary, this separation follows as a necessary consequence. It is 

 simplest to consider the ethereal medium which pervades any transparent 

 substance, together with the material atoms of the substance, as constituting 

 together a compound medium denser than the pure ether, but not more 

 elastic ;* and by comparing the contiguous particles of the rarer and the 

 denser medium with common elastic bodies of different dimensions, we 

 may easily determine not only in what manner, but almost in what degree, 

 this reflection must take place in different circumstances. Thus, if one of 

 two equal bodies strikes the other, it communicates to it its whole motion 

 without any reflection ; but a smaller body striking a larger one is re- 

 flected, with the more force as the difference of their magnitude is greater ; 

 and a larger body, striking a smaller one, still proceeds with a diminished 

 velocity ; the remaining motion constituting, in the case of an undulation 

 falling on a rarer medium, a part of a new series of motions which neces- 

 sarily returns backwards with the appropriate velocity ; and we may 

 observe a circumstance nearly similar to this last in a portion of mercury 

 spread out on a horizontal table ; if a wave be excited at any part, it will 

 be reflected from the termination of the mercury almost in the same 

 manner as from a solid obstacle. 



The total reflection of light, falling, with a certain obliquity, on the 

 surface of a rarer medium, becomes, on both suppositions, a particular case 

 of refraction. In the undulatory system, it is convenient to suppose the 

 two mediums to be separated by a short space in which their densities 

 approach by degrees to each other, in order that the undulation may be 

 turned gradually round, so as to be reflected in an equal angle ; but this 

 supposition is not absolutely necessary, and the same effects may be ex- 

 pected at the surface of two mediums separated by an abrupt termination. 



The chemical process of combustion may easily be imagined either to 

 disengage the particles of light from their various combinations, or to agi- 

 tate the elastic medium by the intestine motions attending it : but the 

 operation of friction upon substances incapable jof undergoing chemical 

 changes, as well as the motions of the electric fluid through imperfect 

 conductors, afford instances of the production of light in which there 



* Some modern writers have adopted the contrary hypothesis, that the ethereal 

 medium which pervades a substance is of the same density as it is in void space, 

 but that its elasticity is different. See Neumann, Memoirs of the Academy of 

 Berlin, vol. xxii. for 1835, and Annalen der Physik, xxv. 418. 



