Mr. Moon on Freshel's T^eo;7/ of Diffraction. 47 



general front (certainly one of the most absurd and chimerical 

 ever proposed) is untrue, and that the whole structure founded 

 upon it as a necessary consequence falls to the ground. But 

 let not the scope of these observations be misunderstood : the 

 principle in question has been represented by some leading 

 writers on this subject as so completely part and parcel of the 

 undulatory theory*, that the student who is little conver- 

 sant with these subjects might suppose, that by rejecting it 

 we reject the theory in toto; but such is by no means the 

 case. 



The principle is quite collateral to the theory. With the 

 investigations into the phaenomena of polarized light it has 

 absolutely no connection whatever. Rejecting it as I do, I 

 still admit and profess my belief in the truth of the undula- 

 tory theory of reflexion and refraction, of the explanations of 

 the interesting experiments of the two mirrors and the prism 

 of small refracting angle, and of that admirable portion of the 

 theory which relates to the phaenomena observed in the sha- 

 dows of narrow fibres and the colours of Newton's rings. 

 But if there be any soundness in the principles I have laid 

 down, the remaining portion of the theory of diffraction, the 

 whole theory of apertures, and of the shadows of extended 

 bodies must be entirely rejected. 



Having thus explained my position, I shall proceed to make 

 some further observations on the subject of diffraction, partly 

 with a view to a more complete exposure of the extraordinary 



* One can hardly resist a smile at the extravagant admiration which 

 some of these gentlemen have displayed towards this monstrous hypothesis. 

 Even were it true that Fresnel's deductions from his principle were correct, 

 that circumstance would only establish its truth, as a curious, indeed, but 

 isolated and useless fact. To call it an explanation were an abuse of terms : 

 for a phaenomenon can only be said to be explained when its occurrence is 

 traced to agents whose existence is known or probable, and the mode of 

 whose operation is understood. Will any one pretend to say that he can 

 understand how a wave should at every point of its progress push out other 

 waves from everj' point of its front ? The mind of an angel could not com- 

 pass such an idea. And if it be admitted that the truth of the proposition 

 cannot be seen a priori, will any be bold enough to assert that they anti- 

 cipate the time when such a property of luminous waves shall be proved ? 

 It is just as likely as that we should be able to prove the existence of the 

 solid epicycles dreamt of by the ancient philosophers. Let nie once for 

 all make a remark which applies as much to Fresnel's theory of double 

 refraction and to tlie labours of others in the same department as to the 

 matter in hand. It is not by a system of hap-hazard conjectures that we 

 are to expect to arrive at truth. After all that has been said and written 

 about the " Inductive Philosophy," one might have expected that by this 

 time its merits were pretty well established, and that we should not now 

 have to restrain the vain efforts of men seeking for light upon any other 

 principles; but in fact, in this case of Fresnel's theory, the whole scientific 

 world seems to have lost sight of these principles. 



