SIX LECTURES ON LIGHT. 



had to establish, by hot conflict, its right to 

 existence. Great names were arrayed against 

 it. Ic had been enunciated by Hooke, it had 

 been applied by lluyghens. it had been de- 

 fended by Euler. But they made no impres- 

 sion. And, indeed, the theory in their hands 

 was more an analogy than a demonstration. 

 It first took the form of a demonstrated verity 

 in the hand of Thomas Young. He broug'ic 

 the waves of light to bear upon each other, 

 causing them to support each other, and to 

 extinguish each other at will. From their 

 mutual actions he determined their lengths, 

 and applied his determinations in all direc- 

 tions. He showed t at the standing difficulty 

 of polarization might be embraced by the 

 theory. After him came Fresnel, whose 

 transcendent mathematical abilities enabled 

 him to give the theory a generality unattained 

 by Young. He grasped the theory in its 

 entirety , followed the ether into its eddies 

 and estuaries in the hearts of crystals of the 

 most complicated structure, and into bodies 

 subjected to strain and pressure. He showed 

 that the facts discovered by Malus, Arago, 

 Brewster, and Biot, were so many ganglia, 

 so to speak, of his theoretic organism, deriv 

 ing from it sustenance and explanation. 

 \Vith a mind too strong for the body with 

 which it was associated, that body became a 

 wreck long before it had become old, and 

 Fresnel died, leaving, however, behind him a 

 name immortal in the annals of science. 



One word more I should like to say regard- 

 ing Fresnel. There are things, ladies and 

 gentlemen, better even than science. There 

 are matters of the character as well as matters 

 of Uu intellect, and it is always a pleasure to 

 those who wish to think well 01 human na- 

 ture, when high intellect and upright charac- 

 ter are combined. They were, I believe, 

 combined in this young Frenchman. In 

 thosi hot conflicts of the undulatory theory, 

 he stood forth as a man of integrity, claiming 

 no more than his right, and ready to concede 

 their rights to others. He at once recognized 

 and acknowledged the merits of Thomas 

 Young. Indeed, it was he, and his fellow- 

 countryman Arago, who first startled England 

 into the consciousness of the injustice done 

 to Young in the Edinburgh Review. 1 should 

 like to read you a brief extract from a letter 

 written by Fresnel to Young in 1824, as it 

 throws a pleasant light upon the character of 

 the Frenc ) philosopher. '* For a long time," 

 says Fresnel, "that sensibility, or that 

 vanity, which people call love of glory, has 

 been much blunted in me. I labor much 

 Jess to catch the suffrages of the public than 

 to obtain that inward approval which has 

 always been the sweetest reward of my efforts. 

 Without doubt, in moments of disgust and 

 discouragement, I have often needed the spur 

 of vanity to excite me to pursue my researches. 

 But all the compliments I have received from 

 Arago, De la Place, and Biot, never gave me 

 so much pleasure as the discovery of a theo- 



retic truth, or the confirmation of a calcu'a- 

 tion by experiment." 



This, ladies and gentlemen, is the core of 

 the whole matter as regards science. It 

 must be cultivated for its own sake, for the 

 pure love of truth, rather than for the ap- 

 plause or profit that it brings. And now my 

 I occupation in America is wellnigh g'7ie". 

 Still I will bespeak your tolerance for a few 

 concluding remarks in reference to the me;i 

 who have bequeathed to us the vast body of 

 knowledge of which I have sought to give 

 you some faint idea in these lectures. What 

 was the motive that spurred them on? what 

 the prize of their high calling for whici they 

 struggled so assiduously? What urged them 

 to those battles and those victories over reti- 

 cent Nature which have become the heritage 

 of the human race? It is never to be for- 

 gotten that not one of those great investiga- 

 tors, from Aristotle down to Stokes and 

 Kirchhoff, had any practical end in view, 

 according to the ordinary definition of the 

 word "practical." They did not propose to 

 themselves money as an end, and knowledge 

 as a means of obtaining it. For the most 

 part, they nobly reversed this process, made 

 knowledge their end, and such money as 

 they possessed the means of obtaining it. 



We may see to-day the issues of their 

 work in a thousand practical forms, and this 

 may be thought sufficient to justify i'c, if not 

 ennoble their efforts. But they did not work 

 for such issues ; their reward wa:, of a totally 

 different kind. We love clothes, we love 

 luxuries, we love fine equipages, we love 

 money, and any man who can point to these 

 as the result of his efforts in life justifies 

 these efforts before all the world. In Ameri- 

 ca and England more especially he is a 

 " practical " man. But I would appeal con- 

 fidently to this assembly whether such things 

 exhaust the demands of human nature? The 

 very presence here for six inclerr.ent nights 

 of this audience, embodying so much oi the 

 mental force and refinement of this great 

 city, is an answer to my question I need 

 not tell such an assembly that there are joys 

 of the intellect as well as joys of the body, 

 or that these pleasures of the spirit consti- 

 tuted the reward of our great investigators. 

 Led on by the whisperings of natural truth, 

 through pain and self-denial, they often pur- 

 sued their work. With the ruling passion 

 strong in death, some of them, when no 

 longer able to hold a pen, dictated to their 

 friends the result of their labors, and then 

 rested from them forever. 



Could we have seen these men at work 

 without any knowledge of the consequences 

 of their work, what should we have thought 

 of them ? To many of their contemporaries 

 it would have appeared simply ridiculous to 

 see men, whose name 3 are now stars in the 

 firmanent of science, straining their atten- 

 tion to observe an effect of experiment al- 

 most too minute for detection. To the un- 



