SIX LECTURES ON LIGHT. 



of the most profound minds that the world j which ?.t any moment constitute the wave, 



Stand upon the sea-shore and observe ihe 



has ever seen ; but he had the misfortune to 

 be too much in advance of his age. He ex- 

 cited the wonder of his contemporaries, who, 

 however, were unab'e to follow him to the 

 heights at which his daring intellect was 



accustomed to soar. His most important j that every particle of water along the front 

 ide s lay, therefore, buried and forgotten in of the wave is in the act of rising, while 

 'the folios of the Royal Society, until a new every particle along its back is in the act of 

 generation gradually and painfully made t^e | sinking. The particles in front reach in suc- 

 same discoveries, and proved the exactness j cession the crest of the wave, and as soon as 



advancing rollers before they are distorted by 

 the friction of the bottom. Every wave has 

 a back and a front, and, if you clearly seiae 

 the image of the moving wave, you will see 



of his assertions and the truth of his demon- 

 strations." 



It is quite true, as Helmholtz says, that 

 Ar oung was in advance of his age ; but some- 

 thing is to be added which illustrates the 

 responsibility of our public writers. Foi 

 twenty years this man of genius was quenched 

 hidden from the appreciative intellect of 

 his countrymen deemed in fact a creamer, 

 liirough the vigorous audacity of a writer 

 who had then possession of the public ear, 

 and who in the Edinburgh Review pourtd 

 ridicule upon Young and his speculations. 

 To the celebrated Frenchmen, Fresnel and 

 Arago, lie was first indebted for the restitu- 

 tion of hu rights, for they, especially Fresnel, 

 remade independently, as Helmholtz says, 

 and vastly extended l.is discoveries. To the 



the crest is passed they begin to fall. They 

 then reac i the furrow or sinus of the wave, 

 and can sink no farther. Immediately after- 

 wards they become the front of the succeed- 

 ing wave, rise again until they reach the 

 crest, and then sink as before. Thus, while 

 the waves pass onward horizontally, the 

 individual particles are simply lifted up and 

 down vertically. Observe a sea-fow], or, if 

 you are a swimmer, abandon yourself to the 

 action cf the waves ; you are not carried for- 

 ward, but simply rocked up and down. The 

 propagation of a wave is the propagation of 

 a form, and not the transference of the sub- 

 stance which constitutes the wave. 



The length of the wave is the d stance 

 from crest to crest, while the distance through 

 which the individual particles oscillate is 



students cf his works Young has long since j called the amplitude of the oscillation. You 

 appeared in his true light, but these twenty will notice that in this description the parti- 

 cles of water are made to vibrate across the 

 line of propagation.* 



blank years pushed him from the public 

 mind, which became in turn filled with the 

 fame of Young's colleague at the Royal In 

 stitution, Davy, and afterwards with the 

 fame of Faraday. Carlyle refers to the re- 

 mark of Novalis, that a man's self-trust is 

 enormously increased the moment he finds 

 that others believe him. If the opposite 

 remark be true if it be a fact that public 

 disbelief weakens a man's force there is no 

 calculating the amount of damage these 

 twenty years of neglect may have done to 

 Young's productiveness as an investigator. 

 It remains to be s:ated thai his assailant was 

 Mr. Henry Brougham, afterwards Lord 

 Chancellor of England. 



Our hardest woik is now before us. And, 

 as I have often had occasion to notice that 

 capacity for hard work depends in a great 

 measure on the antecedent winding up of the 

 will and determination, I would call upon 

 you to gird up your loins for our coming 

 labors. If we succeed in climbing the hill 

 which faces us to-night, our future efforts 

 will be comparatively light. 



In the earliest writings of the ancients we 

 find the notion that sound is conveyed by the 

 air. Aristotle gives expression to this no- 

 tion, and the great architect Vitruvius com- 

 pares the waves of sound to waves of water. 

 But the real mechanism of wave-motion was i 

 hidden from the ancitnts, and indeed was 

 not mad- clear until the time of Newton. 

 The a ntral difficulty of the subject was, to 

 distinguish between the motion of the wave 

 itself and the motion of the particles 



And now we have to take a step forward, 

 and it is the most important, step of ail. You 

 can picture two serie: of waves proceeding 

 from different origins through the same 

 water. When, for example, you throw two 

 stones into still water, the ring- waves pro- 

 ceeding from the two centres of disturbance 

 intersect each other. Now, no mailer how 

 numerous these waves may be. the law holds 

 good that the mo lion of every particle of the 

 water is the algebraic sum of a.l the motions 

 imparted to it. If crest coincide with crest, 

 the wave is lifted to a double height; if fur- 

 row coincide with crest, the motions are in 

 opposition, and thei. sum ij zero. We have 

 then still water, which we shall learn pres- 

 ently corresponds to what we call darkness in 



reference to our present subjs 



This action 



of wave upon wave is technically called in- 

 terference, a term to be remembered. 



Thomas Young's fundamental discovery in 

 optics was that the principle of Interference 

 applied to light. Long prior to his timCj an 

 Italian philosopher. Giimaldi, had stated 

 that, under certain circumstances, two thin 

 beams of light, each of which, acting singly, 

 produced a luminous spot upon a wnite wall, 

 when caused to act together, partially 



* I do not wish to encumber the conception here 

 with the details of the motion, but I inay draw atten- 

 tion to the beautiful model of Professor Lyinao, 

 wherein waves are shown 10 be produc.ed by tho cir- 

 cular motion if the -particles. This, as uroved by 

 the brothers Weber, is the real motley .: :~e case uf 

 water-waves. 



