32 REPORT ON PHYSICAL OPTICS. 



mission," which modify the effects of the attractive and repulsive 

 forces, and in which they are disposed to yield alternately to one 

 or the other. The actual determination of the particle will de- 

 pend, partly on the phase of the fit, and partly on the obliquity 

 under which it meets the bounding surface. Now the molecules 

 composing a beam of light are supposed to be in every possible 

 phase of their fits when they reach the surface : some of them 

 consequently will be reflected, and others refracted ; and the pro- 

 portion of the former to the latter will depend on the incidence. 



As to the fits themselves, Newton thought that they must be 

 referred to a vibratory motion in the ether, excited by the rays 

 themselves, just as a stone flung into water raises waves on its 

 surface. This vibratory motion is supposed to be propagated 

 faster than light itself, and thus to overtake the molecules, and 

 impress upon them the disposition in question by conspiring with 

 or opposing their progressive motion. In one of his queries New- 

 ton has even calculated the lesser limit of the elasticity of the 

 ether, as compared with that of air, in order that it should have 

 so great a velocity of propagation.* The hypothesis of Mr. Mel- 

 ville and M. Biot is more in accordance with the spirit of the 

 theory of emission. The molecules of light are supposed, in this 

 hypothesis, to have a rotatory motion round their centres of gra- 

 vity, which continues along with the progressive motion, and in 

 virtue of which they present attracting and repelling poles alter- 

 nately during their progress in space.f Boscovich imagined a 

 vibratory motion in the parts of the ray itself, which it received 

 at the moment of emission, and retained in its progress.* 



The theory of the fits has now lost much of its credit, since 

 the phenomena of the colours of thin plates phenomena which 

 first suggested it to the mind of Newton have been shown to be 

 irreconcilable with it. The explanation which it gives of the 

 facts now under consideration is, as was observed by Young and 

 Fresnel, inconsistent with the regularity of refraction. In fact, 

 the molecules which are transmitted are not all in the maximum 

 of the fit of transmission, but are supposed to reach the surface in 

 very different phases of this, which may be denominated the posi- 

 tive fit. Now as a change of the fit from positive to negative is, 



* Optics, Query 21. 



t Phil Trans. 1753. Traite de Physique, iv. p. 245. 



\ Philosophies Naturatis Theoria. 



