126 EULOGY ON THOMAS YOUNG. 



EGYPTIAN HIEROGLYPHICS — HISTOIiY OF THE FIRST EXACT INTER- 

 PRETATION GIVEN OF THEM. 



The word hieroglypliic, regarded not inetai)bysically, but iu its natu- 

 ral acceptation, carries us into a field which has been long the theater 

 of numerous and animated debates. I have hesitated whether to risk 



maimer. If the apfrture be otherwise than h)iig with parallel sides, the phcuomenon 

 becomes still more complex, ami the calculation difficult; few such cases have ever yet 

 been solved, and some such cases have been dwelt upon as formidfible objections to 

 the theory ; they are simply cases to Avhich the formula, from its mathematical difiti- 

 cnlties, has not yet been extended. 



In all these cases of diffraction an opaque body was used, and it might still be sus- 

 pected that some action of the edge of that body might be concerned in the result. Nu- 

 merous experiments of Maraldi, Dutour, Biot, and others were directed to the investi- 

 gation of this point. Biot showed that an opaque body was not necessary, inasnmch 

 as the edge of the plate of (/lass, or even the bounding line of two faces of a glass, cut 

 ■ at a slight inclination to each other, gave the same fringes ; indeed, Newton also had 

 noticed something of the kind. Haldat varied the conditions of the edge in every 

 conceivable way, whether of form or nature, by the intluence of magnetism, galvan- 

 ism, electricity,' or temperature from freezing to a red heat, without ])roducing the 

 slio'htest difference iu the fringes — a result which it Avould be impossible to conceive 

 compatible with any idea of an atmosphere of attraction or rej)ul8ion surrounding the 

 edge. 



Again : Thongh we have given the explanation of the external fringes in its simple 

 and correct form, yet both Young and Fresnel failed in the first instance to see it iu 

 that light, both believing that the reflection of a portion of rays from the edge of the 

 opaque body was mainhj concerned in producing the interference. Subsequent experi- 

 ments showed that even in cases where that edge reflects any sensible amount of light, 

 its influence on the diffracted fringes is quite inappreciable. In fact, Young, in a let- 

 ter to Fresnel, in returning thanks for a copy of a later memoir, in which he had shown 

 this supposition to be unnecessary, also concurs in abandoning it. It did but compli- 

 cate and injure the beauty of the result, (Young's Works, i, 393;) and every doubt 

 nmst have disappeared in the minds of those who compared tlie minute arithmetical 

 accuracy with wJiieli the places of the fringes, as computed from the simple theory 

 in the investigations of Fresnel, agreed with those actually determined by the nicest 

 micrometrical measurments. 



In enumerating the discoveries of Young in the first establishment of the wave 

 theory, it is somewhat singular that Arago, whether from accident or design, should 

 have overlooked one investigation which must be regarded as among the most important. 

 The great support which the emission theory received iu recent times was that derived 

 from La]dace's memoir on the law of double refraction, (1809,) iu which, on the prin- 

 ciple of '• least action," as maintained by Maupcu-tius and applied to the idea of lumi- 

 nous molecules, he explained the observed lavrs of ordinary and extraordinary refrac- 

 tion in Iceland spar. This investigation exercised a powerful influence in favor of the 

 molecular theory over the minds of the men of science in France, who bowed iinplicitly 

 to the authority of Laplace. But the memoir of Laplace was the subject of a very 

 powerful attack on the part of Dr. Young, carried on in an article in the Quarterly 

 Review, November, 1809, in which ho disputed the mechanical and mathematical 

 grounds of Laplace's tiieory, and showed that the same laws of double refraction 

 could be far more easily deduced from the undulatory hypothesis. Next to the dis- 

 covery of interference, this refutation of the strongest point of the emission theory 

 cannot but be regarded as one of the most material in the development and establish- 

 ment of the undulatory view. 



To the statement of these various cases of interference it should be added, that when 

 the tints of polarized light were discovered, Y'oung in 1814 applied to the phenomena 

 the general consideration of interference; that is to say, he showed that, owing to 

 the diliering obliquities of the paths of the rays within the crystal, they would 

 be unequally retarded in their passage, and would consequently emerge in con- 

 ditions, with regard to length of route, respectively of accordance or discordance at 

 corresponding distances round the central line or axis of the crystal, and thus might 

 give rise to colored rings. Arago, however, soon noticed that the explanation Avae 

 incomplete ; the main point, in tact, remained to be accounted for, viz, why we see nol 

 colors till the analyser is applied, and why even the previous polarization is necessary 

 to the result. It was not until about two years afterward that Arago and Fresno] 

 jointly succeeded in discovering a new law, which not only furnished tlu^ complete 

 solution of the polarized rings, Imt at length cleared away all the difficulties which, 

 from the first, had surrounded the idea of polarization itself. For an account of this 

 see memoir of Fresnel. — Translator. 



