REPOKT ON PHYSICAL OPTFCS. 307 



posed to yield alternately to one or the other. The actual deter- 

 mination of the particle \vill depend, partly on the phase of the 

 fit, and partly on the obliquity under which it meets the bound- 

 ing 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 proportion of the former to the 

 latter wiU depend on the incidence. 



As to the fits themselves, Newton thought they must be re- 

 ferred 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 Newton has even calculated the lesser limit of the elas- 

 ticity of the ether, as compared with that of air, in order that 

 it should have so great a velocity of propagation*. The hypo- 

 thesis of Mr. Melville 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 gravity which continues along with the progres- 

 sive motion, and in virtue of which they present attracting and 

 repelling poles alternately 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 J. 



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 

 irreconcileable 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 maxi- 

 mum of the fit of transmission, but are supposed to reach the 

 surface in very different phases of this, which may be denomi- 

 nated the positive fit. Now as a change of the fit from positive 

 to negative is, in general, sufficient to overcome altogether the 

 effect of the attractive force, and subject the molecule to the 

 repulsive, it is obvious that the phase of the fit must modify the 

 effects of these forces in every intermediate degree ; and that the 

 molecules which do obey the attractive force must have their 

 velocities augmented in different degrees, depending on the 



* Optics, Query 21. f Phil. Trans. 1753. Traite He Physique, iv. p. 245. 



+ PhilosophitB Naturalis Theoria. 



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