EULOGY ON THOMAS YOUNG. 121 



at cacb of wbicli uo light is reflected from the fihii. This result was of 

 capital importauce ; it included the key to all these phenomena. 



JScwton was less happy in the theoretical views Avhich these remark- 

 able observations suggested to him. To say, with him, that the luminous 

 ray which is reflected is "in a fit of easy reflection;" to say that the ray 

 which passes through the film entire is "in a fit of easy transmission" — 

 what is it but to announce, in obscure terms, merely the same fact which 

 tbe experiment with the two lenses has already taught us '?* 



Tlie theory of Thomas Young is not amenable to this criticism. Here 

 there is no longer admitted any peculiar kind of "fits" as primordial 

 properties of the rays. The thin film is here assimilated in all respects 

 to any thicker reflector of the same substance. If at certain points in 

 its surface no light is visible, Young did not conclude that therefore its 

 reflection had ceased ; he supi)osed that, in the special directions of 

 those points, the rays reflected by the second surface proceeded to meet 

 with those reflected from the first surface, and completely destroyed 

 them. This conflict of the rays is what the author designated by the 

 term '■^interference," which has since become so famous. 



Observe, then, here the most singular of hypotheses. We must cer- 

 tainly feel surprised at finding night in full sunshine at i>oints where 

 tbe rays of that luminary arrive freely; but who would have imagined 

 that we should thence come to supi^ose that darkness could be engen- 

 dered by adding light to light ? 



A physicist is truly eminent when he is able to announce any result 

 which, to such an extent, clashes with all received ideas; but he ought, 

 without delay, to support his views by demonstrative proofs, under tbe 

 I)enalty of being assimilated to those Oriental writers whose fantastic 

 reveries charmed the thousand and one nights of tbe Sultan Schabriar. 



Young bad not this degree of prudence. He showed at once that his 

 theory would agree with tbe phenomena, but without going beyond 

 mere possibility. When at a later period be arrived at real proofs of it, 

 the public bad other prepossessions, which he was not able to overcome. 

 However, tbe experiment, whence our colleague deduced so memorable 

 a discovery, could not excite tbe shadow of a doubt.t 



* In regard to the theory of the "fits," the author liere seems to represent Newton's 

 view as, in fact, mere tautology ; while in other places he is supposed to have indulged 

 in a visionary theory on the subject. Newton, however, expressly says : "What kind 

 of action or disposition this is — whether it consists in a circulating or vibrating 

 motion of the ray, or of the medium, or something else, I do not here inquire." (Oj)- 

 tics, p. 255, ed. 1721.) 



The fact is, Newton in his optical researches expressed the same avowed and syste- 

 matic dislike in indulging in ani) gratuitous theories as in his other inquiries. " Hijpo- 

 iJicics noil Jhif/o,'" was his motto in these as well as in other researches. lu adopting 

 the idea of " fits of easy reiiection and transmission," we are of opinion that hedidnot 

 violate that maxim, and that it was in fact the only legitimate first exiiression of the 

 conclusion whicli the facts warranted. At certain points uo light appeared; it was the 

 legitiinato inference, in the then state of knowledge, tlud none teas rejkctccl. But 

 light was clearly under the same circumstances Iransmitted ; at a distance a little 

 greater along the ray, an opposite etTect was witnessed; and so on. It was nothing 

 more than the strict inference that at those jioints successively soviethituj occurred in the 

 coui-se of the ray which disposed it for, or induced, refiection in the one case, and non- 

 reflection in the other ; accompanied in tlie latter case by like tendency to transmis- 

 sion. These apparent " fits" must be still acknowledged as phenomena ; the mechanism 

 by which they are produced is, however, now known to bo nothing inherent in the 

 light, no essential pro])erty recurring, but the simple periodicity of conspiring or 

 counteracting wa\e action. — Ti:axslator. 



tin the retrospective glance Avhicli the author thus gives over the progress of dis- 

 covery previous to the period at Avhicli Dr. Young first entered on the field, what we 

 liavr, chiefly to observe is, that up to that date nothing like a connected view of the 

 physical character of this wonderful agent had been attained ; a few isolated specula- 



