of light to be infinite, and explained away Romer's observation.
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 ‘pretended demonstration’ which has convinced few of the truth of the proposition, and leaves the question unsolved. Again, in 1690, Huyghens, who in all he wrote had clearer views than Hooke, published his great ‘Traité de la Lumière,’ which was written in 1678. Many of his demonstrations are still completely satisfactory, but on the crucial point he was fatally weak. He, and not Hooke, may claim to be the real founder of the undulatory theory, for he showed what it would do if the rectilinear propagation could only be explained by it. The reasoning of the later pages of Huyghens's first chapter becomes forcible enough when viewed in the light of the principle of interference enunciated by Young on 12 Nov. 1801, and developed by Fresnel in his great memoir on diffraction in 1815; but without this aid it was not possible for Huyghens's arguments to convince Newton, and hence in the ‘Optics’ (2nd ed. 1717) he propounded the celebrated query 28: ‘Are not all hypotheses erroneous in which Light is supposed to consist in pression or motion propagated through a fluid medium?’ ‘If it consisted in pression or in motion propagated either in an instant or in time, it would bend into the shadow. For pression 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 beyond the shadow.’ These were Newton's last words on the subject. They prove that he could not accept the undulatory theory; they do not prove that he believed the emission theory to give the true explanation. And yet the emission theory had done much. Book i. sect. xiv. of the ‘Principia’ treats of the motion of small particles acted on by forces tending towards a body of finite size. The earlier propositions show that if a particle approaching a plane surface be acted on by a force towards the surface, depending only on the distance between the particle and the surface, it will be reflected or refracted according to the known laws of light, and the scholium to prop. xcv. calls attention to the similarity between the particles and light. Such an explanation was first given in the paper of 1675 (Birch, Hist. of Roy. Soc. iii. 256). According to it the particles move more quickly in a dense medium, such as glass or water, than in air; whereas Arago's and Fresnel's experiments in 1819 proved the reverse to be the case, thus verifying Huyghens's views, and upsetting for ever the emission theory (Œuvres Complètes de Fresnel, i. 75). On approaching the surface of a reflecting body the luminous particles are acted on by forces which produce in some cases reflection, in others refraction.
But to explain why some of the incident light is reflected and some refracted Newton had to invent his hypothesis of ‘fits of easy reflection and refraction.’ These are described in the ‘Optics,’ book iii. props. xi., xii., and xiii., 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 surface is put into a certain transient constitution or state, which in the progress of the ray returns at equal intervals, and disposes this ray at every return to be easily transmitted through the next refracting surface, and between the returns to be easily reflected by it.’ ‘Defn. The return of the disposition of any ray to be reflected I will call its Fits of easy reflection, and those of its disposition 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 reflection, some in their Fits of easy transmission.’
Such a theory accounts for some or all of the observed facts. But what causes ‘the fits of easy transmission’? Newton states that he does not inquire, but 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 substances 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, and is in a fit of easy transmission; and, conversely, when the motion of the ray and the wave are opposed, the ray is in a fit of easy reflection. But he was not always so cautious. ‘Were I,’ says he in the ‘Hypothesis’ of 1675, explaining the properties of light (Birch, Hist. of Roy. Soc. iii. 249), ‘to assume an hypothesis it should be this: if propounded more generally so as not to determine what light is farther than that it is something or other capable of exciting vibrations in the æther.’ ‘First, it is to be assumed that there is an æthereal medium. In the second place it is to be supposed that the æther is a vibrating medium like air, only the vibrations far more
