January i6, 1913J 



NATURE 



559 



traverse each other. Or we may arrange the experi- 

 ment as in Fig. ii. If the balls A, and B^ are pushed 

 in the directions of the arrows so as to strike the 

 rows they are shown approaching, the spheres A3 and 

 B3 will spring forward and continue the lines of 

 motion of A^ and B^, and the movements will have 

 ■crossed each other without any injury. He conceived 

 such a result to be beyond explanation on a theory 

 like Newton's. 



His hypothesis met also, as he thought, the other 

 of the two fundamental requirements. The disturb- 

 ance might be supposed to move as fast as was 

 desired, even with the extreme velocity which light, 

 according to Romer, possessed. For, as he says, 

 "there is nothing to hinder us from supposing the 

 particles of the aether to be of a substance as nearly 

 approaching to perfect hardness and possessing a 

 springiness as prompt as we choose." And another 

 very important property of light was illustrated at 

 the same time, viz., that the velocity in free aether 

 was independent of the intensity. 



It is to be observed that Huygens takes the ideas 

 of hardness and impenetrability of matter which he 

 has drawn from the behaviour of glass spheres and 

 applies them to the molecules of the aether. 



From what we have seen of the properties of the 

 new rays we cannot allow Huygens any justification 

 of the reasons which he 

 /~\ gives for his preference 



( ' ) for the wave theory. 



i^ ,^ There were two, you will 



remember. In the first 

 place, he supposed that 

 matter could not move 

 with so great a speed as 

 light ; yet you see that 

 the o particles move 

 practically as fast as he 

 conceived light to move, 

 and they are as mate- 

 rial as anything else. 

 Secondly, he argued that 

 streams of matter could 

 not interpenetrate each 

 ""■ "• other; yet we see that 



atoms can pass through 

 •each other easily. Indeed, the more we consider the 

 behaviour of the rays from radio-active substances, 

 the more impossible appears the view that " parti- 

 cles " of any sort have boundaries which are limits to 

 interpenetration. We see no reason for supposing 

 that there is anything in the known universe which 

 can retain a portion of space to its own exclusive 

 use and forbid all strangers to enter therein. 



So the reasons which Huygens gives for his choice 

 of a hvpothesis are both mistaken ; and we might 

 think that this was a bad beginning for the structure 

 which he built. But his true foundation was laid 

 otherwise. The spreading-pulse theory suggested to 

 him his famous construction of the wave front, which 

 has been of such immense importance in the develop- 

 ment of our knowledge of radiation. His construction 

 gave a correct account of the phenomena of reflection 

 and refraction, and, what was most wonderful, he 

 found himself able to explain by its means the com- 

 plicated motion of light in Iceland spar. In this way 

 fie began the marvellous development of the relations 

 between light and crystalline structure which has 

 roused the interest and admiration of the subsequent 

 centuries. It is true that he had no idea of a regular 

 succession of w-aves ; in fact, he expressly states that 

 he does not wish us to think of his pulses as following 

 each other at regular distances. He did not explain 

 colours, and he failed altogether to account for 

 NO. 2255, VOL. 90] 



polarisation. But his hypothesis linked certain facts 

 together, and was useful so far as it went. 



It w-as Newton with his corpuscular theory who 

 introduced the idea of periodicity in order to explain 

 the colours of the soap film and other "thin plates"; 

 w'ho ascribed differences in colour to differences of 

 frequency, and correctly described the phenomena of 

 polarisation as due to the rays of light having sides, 

 a description which could not be applied to the con- 

 ception of Huygens. Newton was able to express 

 many of the facts known to him in terms of vibrations 

 of an all-pervading aether ; he saw that in such case 

 the longer vibrations would excite the sensation of 

 red, and the shorter — the more refrangible — the sensa- 

 tion of violet. He actually supposed that such vibia- 

 tions travelled along the optic nerves and carri' d the 

 sensations to the brain, and he direrfed attention to 

 persistence of vision as evidence of the " vibratory 

 nature of the motions at the bottom of the eye." 

 Heat he supposed to be conveyed by aether vibrations. 

 He could express the behaviour of a soap film in 

 respect to colour in terms of the wave theory with 

 formal correctness, showing its dependence on the 

 ratio between the thickness of the film and the wave 

 length of light. But he preferred to express his ideas 

 on a corpuscular model, because he could not other- 

 wise explain the formation of sharp shadows, and 

 deemed it impossible for a pulse on rounding a corner 

 to spread so little as light. It was for this reason he 

 rejected the theory of Huygens, and he was perfectly 

 right. If we take Huygens's own model, if we 

 project a billiard ball against a group of other balls 

 in contact, as we might, somewhat inefficiently, start 

 a game of pyramids, the energy of motion is scattered 

 every w"ay, and balls fly in all directions. Huygens 

 never met this objection ; it was not answered until 

 the time of Fresnel, more than a hundred years after- 

 wards. Newton was also impressed with the impos- 

 sibility of varying the nature of light by transmission, 

 reflection, or refraction, and ascribed all apparent 

 changes of colour to sorting processes. As he says, 

 ".very small bodies conserve their properties unchanged 

 in passing through several mediums, which is a con- 

 dition of the rays of light." He was thinking of 

 contemporaries who supposed that the colour of light 

 was readily changed in deviation or transmission. 



The essence of Newton's idea was the travel of 

 light as an entity which did not spread or change 

 as it went. He implied by the term "corporeity of 

 light" no more than "something or other propagated 

 every way in straight lines from luminous bodies 

 without determining what that thing is, whether a 

 confused mixture of difform qualities or modes of 

 bodies, or of bodies themselves; or of any virtues, 

 powers, or beings whatsoever." He strongly opposed 

 a tendencv to read more into his hypothesis than it 

 was intended to hold. In these respects the X-ray 

 resembles very closely the corpuscle which Newton 

 conceived, so long at least as it remains untrans- 

 formed. But if transformation occurs the electron 

 generally loses energ\-, and a retransformed X-ray 

 will have less energy than the original, a well-know'n 

 process. It may be compared with the phenomenon 

 of fluorescence, of w'hich Newton knew nothing. 



Now- if X-rays are to be classed with light, as there 

 are reasons for supposing, and as many do suppose 

 with more or less conviction, then it must be acknow- 

 ledged that Newton's conception has more value in it 

 than the last century has been accustomed to grant. 

 But we shall not therefore adopt Newton's theorv as 

 he left it. It is too obviously defective. It cannot 

 explain diffraction, and his main reason for reiecting 

 the wave theory was wrong. He gave no satisfactory 

 explanation of .the uniformity of the velocity of light 



