568 Professor Tyndall [Jan. 22, 



Fresnel, entered the field. He presented in 1815, to the French 

 Institute, a memoir on Diffraction, which marks an epoch in the 

 history of the wave theory. It is usual, when such a paper is pre- 

 sented, to refer it to a " Commission " who consider it and report upon 

 its merits. The commissionaires in this instance were Arago and 

 Prony. 



Arago had read the memoirs of Young in the ' Philosophical 

 Transactions,' but had not understood their full significance. The 

 study of Fresnel's memoir caused the full truth to flash upon him 

 that his young countryman had been anticipated thirteen years pre- 

 viously by Dr. Young, Fresnel had re-discovered the principle of 

 interference independently, and had applied it, with profound insight 

 and unrivalled experimental skill, to the phenomena of diffraction. 

 It was no light thing to Fresnel to find himself, as regards the 

 principle of interference, suddenly shorn of his glory. He, however, 

 bore the shock with resignation. He might have readily made claims 

 which would have found favour with his countrymen and with the 

 world at large. But he did nothing of the kind. The history of 

 science, indeed, furnishes no brighter example of honourable fairness 

 than that exhibited throughout his too short life by the illustrious 

 young Frenchman. Once assured that he had been anticipated — what- 

 ever might have been the extent of his own labours, however in- 

 dependently he might have arrived at his results, he unreservedly 

 withdrew all claim to the discovery. There is, I repeat, no fairer 

 example of scientific honour than that manifested by Augustin 

 Fresnel. 



Fresnel was a powerful mathematician, and well versed in the 

 best mathematical methods of his day. With enormous labour he 

 calculated the positions where the phenomena of interference must 

 display themselves in a definite way. He was, moreover, a most 

 refined experimentalist, and having made his calculations, he devised 

 instrumental means of the most exquisite delicacy, with the view of 

 verifying his results. In this way he swept the field of diffraction 

 practically clear of difficulty, solving its problems where even Young 

 had failed. 



Truly, these were minds possessing gifts not purchasable with 

 money ! and round about the central labours of both of them, minor 

 achievements of genius are to be found, which would be a fortune 

 to less opulent men. I hardly know a finer example of Young's 

 penetration than his account of the spurious or supernumerary 

 bows, observed within the true primary rainbow. These interior 

 bows are produced by interference. It is not difficult, by artificial 

 means, to form these bows in great number and beauty. This is 

 a subject on which, as you are aware, I worked a couple of years ago 

 myself. And often, when looking at these bows, the words of Young 

 seemed to me like the words of proiDhecy. The bows were the 

 physical transcript of what Young stated must occur ; a transcript, 

 moreover, which, when compared with his words, was far more com- 



