.jj),^ PROFESSOR KF.LLANI) ON f'RESNKI/S FORMULAE FOR THE 



turc and results of the molecuhxr hypothesis, I return to the examination of the 

 memoir, which appears in some points to arffuc against it. 



Mr (iREEN states, that two waves will result from giving a motion to a fluid, 

 sueh as that commonly supposed to be the medium the vibrations of which con- 

 stitute light, the one transversal and the otiier normal. On a careful examina- 

 tion of his memoir, I cannot discover this normal vibration; the nearest approach 

 to it appears to result from the circumstance, that two waves, the incident and 

 the retiected, may be transmitted at the same tune, and therefore cross each other. 

 If. then, in this case, the angle of incidence l)e an angle of 45°, one vibration may 

 l)e at right angles to the other ; but this circumstance does not in the slightest 

 degree militate against any conclusions which have been arrived at by the mole- 

 cular hypothesis. The coexistence of vibrations travelling in different directions, 

 is distinctly recognised in that theory. It may be well to state clearly, that the 

 point, and I think a most important one, which has been proved from the mole- 

 cular hypothesis, is this ; that one wave cannot consist partly of normal, partly 

 I )f transversal vibrations. Of coiirse, the definition of the wave restricts it to a 

 state of motion transmitted in one direction with one velocity. 



There can be little doubt, however, that the normal vibration to which Mr 

 Green refers, is supposed to lie contained in that function which he introduces 

 in the body of his memoir, as the result of the change of motion from an incident 

 and retiected to a refracted one. This viljration is, however, merely a vibrator^^ 

 motion, not transmitted in the same direction as the incident ; and in the sequel 

 of the present memoir, it will appear that it is really and bondjidc a transverse 

 vibration. Thus a statement, which at the first sight appears to argue powerful- 

 ly against the molecular theory, does, when attentively examined, afford strong 

 presumptive evidence in its favour. 



I have deemed it right to be explicit on this subject, as the admission of Mr 

 Green's statement, if it left hypotheses such as Laplace's as to the constitution 

 of media uninjured, would absolutely crush the more probable hypothesis of the 

 Newtonian law of gravitation applied to the ultimate atoms. 



There is another point in Mr Green's paper which, although not so im- 

 portant as the one just noticed, will require an answer of a very different natiu-e. 

 and ought consequently to be attended to. It is this : in order to obtain the law 

 which Fresnel has deduced for the intensity of hght polarized in the plane of 

 incidence, it is found requisite to assume that the velocity of transmission varies 

 inversely as the square root of the density. 



Tiiis overthrows, apparently, aU the previous conclusions of the molecular 

 hypothesis ; for all its advocates, as far as I recollect, have come to the conclu- 

 sion that the density of the caloric -within refracting media is less than it is in 

 nicMo. But it is desu-able that great caution should be exercised in judging of 

 this and like apparent oppositions. We have no very precise notion of the pro- 



