September 14, 1893J 



NA TURE 



475 



I elastic. ... In the second place, it is to be supposed that the 

 ether is a vibrating medium, like air, only the vibrations far 

 more swift and minute ; those of air made by a man's ordinary 

 voice succeeding at more than half a foot or a foot distance, 

 but those of ether at a less distance than the hundred-thousandth 

 part of an inch. And as in air the vibrations are some larger 

 j than others, but yet all equally swift ... so I suppose the ethereal 

 ( vibrations differ in bigness but not in swiftness. ... In the 

 I fourth place, therefore, I suppose that light is neither ether nor 

 j its vibrating motion, but something of a different kind propa- 

 j gated from lucid bodies. They that will may suppose it an 

 I aggregate of various peripatetic qualities. Others may suppose 

 • it multitudes of unimaginable small and swift corpuscles of 

 various sizes springing from shining bodies at great distances 

 I one after the other, but yet without any sensible interval of 

 ■time. . . . To avoid dispute and make this hypothesis general, 

 j let every man here take his fancy ; only, whatever light be, I 

 would suppose it consists of successive rays differing from one 

 ■ another in contingent circumstances, as bigness, force, or vigour, 

 I like as the sands on the shore . . . and, further, I would sup- 

 1 pose it diverse from the vibrations of the ether. . . . Fifthly, 

 lit is to be supposed that light and ether mutually act upon one 

 another." It is from this action that reflexion and refraction 

 come about ; " aethereal vibrations are therefore," he continues, 

 " the best means by which such a subtile agent as light can 

 ishake the gross particles of solid bodies to heat them. And so, 

 'supposing that light impinging on a refracting or reflecting 

 ethereal superficies puts it into a vibrating motion, that phy- 

 sical superficies being by the perpetual appulse of rays always 

 kept in a vibrating motion, and the ether therein continually 

 ^expanded and compressed by turns, if a ray of light impinge on 

 jit when it is much compressed, I suppose it is then too dense 

 and stiff to let the ray through, and so reflects it ; but the rays 

 :hat impinge on it at other times, when it is either expanded by 

 .he interval betw een two vibrations or not loo much compressed 

 ind condensed, go through and are refracted. . . . And now 

 o explain colours. I suppose that as bodies excite sounds of 

 .arious tones and consequently vibrations, in the air of various 

 :iignesses, so when the rays of light by impinging on the stiff 

 efracling superficies excite vibrations in the ether, these rays 

 :xcite vibrations of various bignesses . . . therefore, the ends 

 if the capillamenta of the optic nerve which front or face the 

 etina being such refracting superficies, when the rays impinge 

 '■n them ihey must there excite these vibrations, which vibra- 

 ions (like those of sound in a trumpet) will run along the 

 queous pores or crystalline pith of the capillamenta through 

 le optic nerves into the sensorium (which light itself cannot 

 o), and there, I suppose, affect the sense with various colours, 

 ccording to their bigness and mixture — the biggest with the 

 rongest colours, reds and yellows ; the least with the weakest, 

 lues and violets ; the middle with green ; and a confusion of 

 1 with white." 



The last idea, the relation of colour to the bigness of wave- 

 ngth, is put even more plainly in the "Opticks," Query 13 

 d. 1704) : — " Do not several sorts of rays make vibrations of 

 irious bignesses, which according to their bignesses excite 

 nsations of various colours . . . and, particularly, do not the 

 est refrangible rays excite the shortest vibrations for making 

 sensation of deep violet ; the least refrangible the largest for 

 aking a sensation of deep red ? " 



The whole is but a development of a reply, written in 1672, 



a criticism of Hooke's on his first optical paper, in which 



LWton says : " It is true that from my theory I argue the cor- 



>reity of light, but I do it without any absolute posiliveness, 



j the word perhaps intimates, and make it at most a very 



lausible consequence of the doctrine, and not a fundamental 



pposition. Certainly," he continues, " my hypothesis has a 



ich greater affinity with his own [Hooke's] than he seems to be 



:ire of, the vibrations of the ether being as useful and neces- 



y in this as in his." 



iThus Newton, while in the "Opticks" he avoided declaring 



juself as to the mechanism by which the fits of easy reflexion 



*d transmission were produced, has in his earlier writings 



sveloped a theory practically identical in many respects with 



|)dern views, though without saying that he accepted it. It 



<s an hypothesis ; one difficulty remained, it would not 



irount for the rectilinear propagation, and it must be rejected 



^ it did. 



Light is neither ether nor its vibrating motion ; it is energy 



»ich, emitted from luminous bodies, is carried by wave motion 



in rays, and falling on a reflecting surface sets up fresh waves 

 by which it is in part transmitted and in part reflected. Light 

 is not material, but Newton nowhere definitely asserts that it is. 

 He "argues the corporeity of light, but without any absolute 

 positiveness. " In the " Principia," writing of his particles, his 

 words are: " Harum attractionum baud multum dissimilessunt 

 Lucis reflexiones et refractiones " ; and the Scholium concludes 

 with " Igitur ob analogiam quas est inter propagationem radio- 

 rum lucis et progressum corporum, visum est propositiones 

 sequentes in usus opticossubjungere ; interea de natura radio- 

 rum (utrum sint corpora necne) nihil omnino disputans, sed tra- 

 jectorias corporum trajectoriis radiorum persimiles solummodo 

 deterrainans. " ^ 



No doubt Newton's immediate successors interpreted his 

 words as meaning that he believed in the corpuscular theory, 

 conceived, as Herschel says, by Newton, and called by his 

 illustrious name. Men learnt from the "Principia" how to 

 deal with the motion of small particles under definite forces. 

 The laws of wave motion were obscure, and till the days of 

 Young and Fresnel there was no second Newton to explain 

 them. There is truih in Whewell's words ("Inductive 

 Sciences," ii. chap, x.): "That propositions existed in the 

 'Principia' which proceeded on this hypothesis was with 

 many ground enough for adopting the doctrine." Young's 

 view, already quoted, a pi ears to me m»re just ; and I see in 

 Newton's hypothesis the first clear indication of the undulatory 

 theory of light, the first statement of its fundamental laws. 



Three years later (1678) Huygens wrote his " Traite de la 

 Lumiere," published in 1690. He failed to meet the main 

 difficulty of the theory, but in other respects he developed its 

 consequences to a m)St remarkable degree. For more than a 

 century after this there was no progress, until in 1801 the prin- 

 ciple of interference was discovered by Young, and again 

 independently a few years later by Fresnel, whose genius 

 triumphed over the difficulties to which his predecessors had 

 succuiibed, and, by combining the principles of interference 

 and transverse vibrations, established an undulatory theory as a 

 fact, thus making Newton's theory a vera causa. 



There is, however, a great distinction between the emission 

 theory as Newton left it and Fresnel's undulatory theory. The 

 former was dynamical, though it could explain but little : the 

 particles of light obeyed the laws of motion, like particles of 

 matter. The undulatory theory of Huygens and Fresnel was 

 geometrical or kinematical : the structure of the ether was and 

 is unknown ; all that was needed was that light should be due 

 to the rapid periodic changes of some vector property of a me- 

 dium capable of transmitting transverse waves. Fresnel, it is 

 true, attempted to give a dynamical account of double refraction, 

 and of the reflexion and refraction of polarised light, but the 

 attempt was a failure ; and not the least interesting part of Mr. 

 L. Fletcher's recent book on double refraction ("The Optical 

 Indicatrix ") is that in which he shows that Fresnel himself in 

 the first instance arrived at his theory by purely geometrical 

 reasoning, and only attempted at a later date to give it its dy- 

 namical form. "If we reflect," says Stokes (" Report on Double 

 Refraction," Brit. Assoc Reforl, 1862, p. 254), "on the stale of 

 the subject as Fresnel found it and as he left it, the wonder is, 

 not that he failed to give a rigorous dynamical theory, but that 

 a single mind was capable of effecting so much." Every student 

 of optics should read Fresnel's great memoirs. 



But the time was coming when the attempt to construct a 

 dynamical theory of light could be made. Navier, in 1821, 

 gave the first mathematical theory of elasticity. He limited 

 himself to isotropic bodies, and worked on Boscovitch's hypo- 

 thesis as to the constitution of matter. Poisson followed on the 

 same lines, and the next year (1822) Cauchy wrote his first 

 memoir on elasticity. The phenomena of light afforded a 

 means of testing this theory of elasticity, and accordingly the 

 first mechanical conception of the ether was that of Cauchy and 

 Neumann, who conceived it to consist of distinct hard particles 

 acting upon one another with forces in the line joining them, 

 which vary as some function of the distances between the par- 

 ticles. It was now possible to work out a mechanical theory 

 of light which should be a necessary consequence of these hy- 



1 The reflexions and refractions of light are not very unlike these attrac - 

 tions. Therefore, because of ihe analogy which exists between the propa- 

 gation of rays of light and the molion of bodies, it seemed right to add the 

 following propositions for optical purposes, not at all with any view of 

 discussing the nature of rays (whether they are corporeal or not), but only to 

 determine paths of particles which closely resemble the paths of rays. — 

 " Principia," lib. i., sect, xiv., prop, xcvi., Scholium. 



NO. 1246, VOL. 48] 



