10 



SIX LECTURES ON LIGHT. 



times slow, sometimes rapid, and at all times 

 a source of intellectual joy. When rapid, the 

 pleasure is concentrated and becomes a kind 

 of ecstasy or intoxication. To any one who 

 has experienced this pleasure, even in a mod- 

 erate degree, the action of Archimedes when 

 he quitted the bath, and ran naked, crying 

 " Eureka! " through the streets of Syracuse, 

 becomes intelligible. 



How, then, did it fare with the theory cf 

 Newton, when the deductions from it were 

 brought face to face with natural phenomena ? 

 To the mind's eye, Newton's elastic particles 

 present themselves like particles of sensible 

 magnitude. The same reasoning applies to 

 both ; the same experimental checks exist for 

 both. Tested by experiment, then, Newton's 

 theory was found competent to explain many 

 facts, and with transcendent ingenuity its 

 author sought to make it account for all. He 

 so far succeeded, that men so celebrated as 

 Laplace and Malus, who lived till 1812, and 

 13iot and Brewster, who lived till our own 

 lime, were found among his disciples. 



Still, even at an early period of the existence 

 of the Emission Theory, one or two great 

 names were found recording a protest against 

 it ; and they furnish another illustration of 

 the law that, in forming theories, the scientific 

 imagination must draw its materials from the 

 world of fact and experience. It was known 

 long ago that ; ound is conveyed in waves or 

 puses through the air ; and no sooner was this 

 truth well housed in the mind than it was trans- 

 formed into a theoretic conception. It was 

 supposed that light, like sound, might also be 

 the product of wave-motion. But what, in 

 this case, could be the material forming the 

 waves ? For the waves of sound we have the 

 air cf our atmospheie ; but the stretch of im- 

 agination which filled all space with a luminif- 

 erous ether trembling with the waves of light 

 was so bold as to shock cautious minds. In 

 one of my latest conversations with Sir David 

 Brewster he said to me that his chief objection 

 to the undulatory theory of light was that he 

 could not think the Creator guilty of so clumsy 

 a contrivance as the filling of space with ether 

 in order to produce light. This, I may say, 

 is very dangerous ground, and the quarrel of 

 science with Sir David, on this point, as with 

 many other persons on other points, is, that 

 they profess to know too much about the 

 mind of the Creator. 



This conception of an ether was advocated 

 and indeed applied to various phenomena of 

 optics by the celebrated astronomer, Huy- 

 gh^ns. It was espoused and defended by the 

 celebrated mathematician, Euler. They were, 

 however, opposed by Newton, whose authority 

 at -lie time bore them down. Or shall I say 

 it was authority merely ? Not quite so. 

 Newton's preponderance was in some degree 

 due to the fact that, though IIuyghe::s and 

 Euler were right in the main, t..ey did not 

 oossess sufficient data to prove themselves 

 right. No human authority, however high, 



can maintain itself against the voice of Nature 

 speaking through experiment. But the voice 

 of Nature may be an uncertain voice, through 

 the scantiness of data. This was the case at 

 the period now referred to, and at such a pe- 

 riod by the authority of Newton all antago- 

 nists were naturally overborne. 



Still, this great Emission Theory, which 

 held its ground so long, resembled one of 

 those circles which, according to your coun- 

 tryman Emerson, the force of genius periodi- 

 cally draws round the operations of the in- 

 tellect, but which are eventually broken 

 through by p es^ure from behind. In the 

 year 1773 was born, at Milverton, in Somer- 

 setshire, one of the most remarkable men that 

 England ever produced. He was educated 

 for the profession of a physician, but was too 

 strong to be tied down to professional routine. 

 He devoted himself to the study of natural 

 philosophy, and became in all its departments 

 a master. He was also a master of letters. 

 Languages, ancient and modern, were housed 

 within his brain, and, to use the words of his 

 epitaph, "he first penetrated the obscurity 

 which had veiled for ages the hieroglyphics of 

 Egypt." It fell to the lot of this man to dis- 

 cover facts in optics which Newton's theory 

 was incompetent to explain, and his mind 

 roamed i* search of a sufficient theory. He 

 had made himself acquainted with all the 

 phenomena of wave-moti ,n ; with all the 

 phenomena of sound ; working successfully 

 in this domain as an original discoverer. 

 Thus informed and disciplined, he was pre- 

 pared to detect any resemblance which might 

 reveal itself between the phenomena of light 

 and those of wave-motion. Such resem- 

 blances he did detect ; and, spurred on by 

 the discovery, he pursued his speculations and 

 his experiments, until he finally succeeded in 

 placing on an immovable basis the Undulatory 

 Theory of Light. 



The founder of this great theory "yas 

 Thomas Young, a name, perhaps, unfamiliar 

 to many of you. Permit me, by a kind of 

 geometrical construction which I once em- 

 ployed in London, to give you a notion of the 

 magnitude of this man. Let Newton stand 

 erect in his age, and Young in his. Draw a 

 straight line from Newton to Youn^,, which 

 shall form a tangent to the heads of both. 

 This line would slope downwards from New- 

 ton to Young, because Newton was certainly 

 the taller man of the two. But the slope 

 would not be steep, for the difference of stat- 

 ure was not excessive. The line would form 

 what engineers call a gentle gradient from 

 Newton to Young. Place underneath this 

 line the biggest man born in the interval 

 between both. Pie would not, in my opinion, 

 reach the line ; for if he did he would be 

 taller intellectually than Young, and there 

 was, I believe, none taller. But I do not 

 want you to rest on English estimates of 

 Young; the German, Helmholtz, a kindred 

 jenius, thus speaks of him : " His was one 



