474 



NATURE 



[September 14, 1893 



wrote pure nonsense in the attempt ; and though his writings 

 contain the germ of the theory, and in the light of our present 

 knowledge it seems possible that he understood it more 

 thoroughly than his contemporaries believed, yet his reasoning 

 is so utterly vague and unsatisfactory that there is little ground 

 for surprise that he convinced but few of its truth. 



And then came N'ewtnn. It is claimed for him, and that 

 with justice, that he was the true founder of the emission theory. 

 In Descartes' hands it was a vague hypothesis. Newton de- 

 duced from it by rigid reasoning the laws of reflexion and re- 

 fraction ; he applied it with wondrous ingenuity to explain the 

 colours of thin and of thick plates and the phenomena of diffrac- 

 tion, though in doing this he had to suppose a mechanism 

 which he must have felt to be almost impossible ; a mechani-m 

 which in time, as it was applied to explain other and more 

 complex phenomena, became so elaborate that, in the words of 

 Verdet, referring to a period one hundred years later, "all that 

 is necessary to overturn this laborious scaffolding is to look at 

 it and try to understand it." 



But though Newton may with justice be called the founder 

 of the emission theory, it is unjust to his memory to state that 

 he accepted it as giving a full and satisfactory account of optics 

 as they were known to him. When he first began his optical 

 work he realised that facts and measurements were needed, and 

 his object was to furnish the facts. He may have known of 

 Hooke's theories. The copy of the " Micrographia" now at 

 Trinity College was in the Library while Newton was working 

 with his prism in rooms in college, and may have been con- 

 sulted by him. An early note-book of his contains quotations 

 from it. Still there was nothing in the theories but hypotheses 

 unsupported by facts, and these would have no charm for 

 Newton., The hypotheses in the main are right. Light is due 

 to wave motion in an all-pervading ether ; the principle of 

 interference, vaguely foreshadowed by Hooke (" Micro- 

 graphia," p. 66), was one which a century later was to remove 

 the one difficulty which Newton felt. For there was one fact 

 which Hooke's theory could not then explain, and till that ex- 

 planation was given the theory must be rejected ; the test was 

 crucial, the answer was decisive. 



Newton tells us repeatedly what the difficulty was. In reply 

 to a criticism of Hooke's in 1672, he writes : " For to me the 

 fundamental supposition itself seems impossible, namely, that 

 the waves of vibrations of any fluid can, like the rays of light, 

 be propagated in straight lines without continual and very ex- 

 travagant spreading and bending into the quiescent medium 

 where they are terminated by it. I mistake if there be not 

 both experiment and demonstration to the contrary. . . . For 

 it seems impossible that any of those motions or pressions can 

 be propagated in straight lines without the like spreading every 

 way into the shadowed medium." 



Nor was there anything in the controversy with Hooke, 

 which took place about 1675, to shake this behef Hooke had 

 read his paper describing his discovery of diffraction. He had 

 announced it two years earlier, and there is no doubt in my 

 mind that this was an original discovery, and not, as Newton 

 seemed to imply soon after, taken from Grimaldi ; but his paper 

 does not remove the QifSculty. Accordingly we find in the 

 "Principia" Newton's attempted proof (lib. ii. prop. 42) that 

 "motus omnis per fluidum propagatus divergit a recto tramite 

 in spatia immota "—a demonstration which has convinced but 

 few and leaves the question unsolved as before. 



Again, in 1690 Huygens published his great " Traite de la 

 Lumiere," written in 1678. Huygens had clearer views than 

 Hooke on all he wrote ; many of his demonstrations may be 

 given now as completely satisfactory, but on the one crucial 

 matter he was fatally weak. He, rather than Hooke, is the 

 true founder of the undulatory theory, for he showed what it 

 would do if it could but explain the rectilinear propagation. 

 The reasoning of the latter part of Huygens's first chapter be- 

 comes forcible enough when viewed in the light of the principle 

 of interference enunciated by Young, November 12, 1801, and 

 developed, independently of Young, by Fresnel in his great 

 memoir on " Diffraction" in 1815 ; but without this aid it was 

 not possible for Huygens's arguments to convince Newton, and 

 hence in the "Opticks" (2nd edit., 1717) he wrote the cele- 

 brated Query 28 : "Are not all hypotheses erroneous in which 

 light is supposed to consist in pressure or motion propagated 

 through a fluid medium? If it consisted in motion propagated 

 either in an instant or in time it would bend into the shadow. 

 For pressure or motion cannot be propagated in a fluid in right 



lines beyond an obstacle which stops part of the motion, but 

 will bend and spread every way into the quiescent medium 

 which lies out.side the shadow." These were his last words on 

 the subject. They prove that he could not accept the undu- 

 latory theory ; they do not prove that he believed the emission 

 theory to give the true explanation. Yet, in spite of this, I 

 think that Newton had a clearer view of the undulatory theory 

 than his contemporaries, and saw more fully than they did what 

 that theory could achieve if but the one difficulty were 

 removed. ♦^ 



This was Young's belief, who writes [Phil. Trans., No- 

 vember 12, 1801) : — " A more extensive examination of 

 Newton's various writings has shown me that he was in reality 

 the first who suggested such a theory as I shall endeavour to 

 maintain ; that his own opinions varied less from this theory 

 than is now almost universally believed ; and that a variety of 

 arguments have been advanced as if to meet him which may be 

 found in a nearly similar form in his own works." I wish to 

 call attention to this statement, and to bring into more pro- 

 minent view the grounds on which it rests, to place Newton in 

 his true position as one of the founders of the undulatory 

 theory. 



The emission theory in Newton's hands was a dynamical 

 theory ; he traced the motion of material particles under certain 

 forces, and found their path to ciincide with that of a ray of 

 light ; and in the " Principia," prop, xcvi.. Scholium, he calls 

 attention to the similarity between these panicles and -ight. 

 The particles obey the laws of reflexion and refraction ; but to 

 explain why some of the incident light was reflected and some 

 refracted Newton had to invent his hypothesis of fits of easy 

 reflexion and transmission. These are explained in the 

 "Opticks," book iii., props, xi., xii., and xiii. (1704), thus : — 



"Light is propagated from luminous bodies in time, and 

 spends about seven or eight minutes of an hour in passing from 

 the sun to the earth. 



" Every ray of light in its passage through any refracting sur- 

 face is put into a certain transient constitution or state, which 

 in the progress of the ray returns at equal intervals, and dis- 

 poses the ray at each return to be easily transmitted through 

 the next refracting surface, and between the returns to be easily 

 reflected by it. 



" Dejitiition. — The return of the disposition of any ray to be 

 reflected I will call its fit of easy reflexion, and those of the dis- 

 position to be transmitted its fits of easy transmission, and the 

 space it passes between every return and the next return the 

 interval of its fits. 



"The reason why the surfaces of all thick transparent bodies 

 reflect part of the light incident on them and refract the rest is 

 that some rays at their incidence are in their fits of easy 

 reflexion, some in their fits of easy transmission." 



Such was Newton's theory. It accounts for some or all of 

 the observed facts ; but what causes the fits ? Newton, in the 

 "Opticks," states that he does not inquire; he suggests, for 

 those who wish to deal in hypotheses, that the rays of light 

 striking the bodies set up waves in the reflecting or refracting 

 substance which move faster than the rays and overtake them. 

 When a ray is in that part of a vibration which conspires with 

 its motion, it easily breaks through the refracting surface — it is 

 in a fit of easy transmission ; and, conversely, when the motion 

 of the ray and the wave are opposed, it is in a fit of easy 

 reflexion. 



But he was not always so cautious At an earlier date (167S) 

 he sent to Oldenburg, for the Royal Society, an "Hypothesis 

 explaining the Properties of Light " ; and we find from the 

 journal book that "these observations so well pleased the 

 society that they ordered Mr. Oldenburg to desire Mr. Newton 

 to permit them to be published." Newton agreed, but asked 

 that publication should be deferred till he had completed the 

 account of some other experiments which ought to precede 

 those he had described. This he never did, and the hypothesis 

 was first printed in Birch's " History of the Royal Society," 

 vol. iii., pp. 247, 262, 272, &c. ; it is also given in Brewster's 

 "Life of Newton," vol. i., App. II., and in the Phil. Mag., 

 September, 1846, pp. i87-2t3. 



" Were I," he writes in this paper, " to assume an hypothesis, 

 it should be this, if propounded more generally, so as not to 

 assume what light is further than that it is something or other 

 capable of exciting vibrations of the ether. First, it is to be 

 assumed that there is an ethereal medium, much of the same 

 constitution with air, but far rarer, subtiller, and more strongly 



NO. 1246, VOL. 48] 



