112 



KNOWLEDGE 



[May i, 18§4. 



that inanimate brute matter should, without the mediation 

 of something else which is not material, operate on and 

 affect other matter without mutual contact, as it must do 

 if gravitation, in the sense of Epicurus, be essential and 



inherent in it That gravity should be innate, 



inherent, and essential to matter, so that one body may 

 act upon another at a distance through a racuiiin, without 

 the mediation of anything else by and through which 

 their action and force may be conveyed from one to another, 

 is to me so great an absurdity that I believe no man who 

 has in philosophical matters a competent faculty of thinking 

 can ever fall into it. Gravity must be caused by an agent 

 acting constantly according to certain laws, but whether 

 this agent be material or immaterial I have left to the 

 consideration of my readers." 



Newton explained the phenomena of light as due to 

 luminous corpuscles emitted with great velocity from 

 shining bodies. He rejected the undulatory theory because 

 he could not explain by it the propagation of light in 

 straight lines, but some of his ideas closely resemble and 

 agree with those of the theory of undulations ; and he also 

 supposed the existence of an all-pervading ether, and used 

 the undulatory theory to account for the occurrence of 

 reflection and refraction. 



He imagined that his luminous corpuscles on striking 

 surfaces produced waves in the ether, and by the 

 action of these waves alternate fits of easy reflection and 

 easy transmission were communicated to the luminous par- 

 ticles, so that sometimes they were in a state to be reflected 

 and sometimes in a condition to be refracted at a trans- 

 parent surface. He applies his observations on the colours 

 of thin plates to this hypothesis, and develops it with extra- 

 ordinary ingenuity. Many of his notions as to the waves 

 produced in the ether agree far more closely with the undu- 

 latory theory than has been generally supposed. He com- 

 pares the waves in the ether to those produced on throwing 

 a stone into water. He says : "What kind of action or 

 disposition this is — whether it consists in a circulating or 

 a vibrating motion of the ray, or of the medium, or some- 

 thing else — I do not here inquire." He considered that 

 vibrations or tremors were excited in the reflecting or 

 refracting medium at the point of incidence, that they were 

 propagated to great distances, and that they overtook the 

 rays of light and successively put them into the fits of 

 easy reflection and easy transmission. In the queries at 

 the end of his Opticks, he asks: "Is not the heat of the 

 warm room conveyed through the vacuum by the vibrations 

 of a much subtler medium than air ? And is not this 

 medium the same with that medium by which light is 

 refracted and reflected, and by whose vibrations light 

 communicates heat to bodies, and is put into fits of easy 

 reflection and easy transmission?" He gave as a possible 

 explanation for gravitation that the ether was much rarer 

 within the dense bodies of the sun, stars, and planets than 

 in the spaces between them ; that it grew denser and 

 denser as the distance from these bodies increased, and 

 thereby caused the gravitation of the bodies towards each 

 other, every body tending to go from the denser parts of 

 the medium towards the rarer. Again he asks : " Is not 

 vision performed chiefly by the vibrations of this medium 

 excited in the bottom of the eye by the rays of light, and 

 propagated through the solid, pellucid, and uniform 

 capillamenta of the optic nerves into the place of 

 sensation ? " 



The ether must be of extreme tenuity and imponderable. 

 Its presence cannot be detected by our senses, and its 

 properties must be discovered by a process of reasoning 

 founded on its behaviour, as manifested, for example, in 

 the transmission of luminous vibrations. Its properties 



are to a great extent at present unknown. We know that it 

 must fill the celestial spaces as far as the most distant stars ; 

 and from the propagation of light through transparent 

 material we judge that the ether must evidently interpene- 

 trate solids and fluids, for the matter of the solid body itself 

 is incapable of transmitting vibrations with the enormous 

 rapidity of those of light. From phenomena such as 

 the polarization of light it is seen that the direction of 

 the vibrations must be transverse to the direction of 

 propagation. 



As fluids cannot give rise to transverse vibrations, 

 the ether was supposed to behave like an elastic solid, or at 

 any rate to possess some property analogous to rigidity or 

 resistance to change of shape. As a first step, the vibrations 

 constituting light were regarded as actual periodic displace- 

 ments of the particles of the ether in the wave front, 

 and transverse to the direction in which the wave is 

 moving. A solid gives rise to longitudinal vibrations as 

 well as to transverse ones, and to get rid of the complica- 

 tion arising from the existence of the former the ether 

 was supposed incompressible ; then the velocity of 

 propagation of the longitudinal disturbance would be 

 infinite. Sir G. Stokes has pointed out that though the 

 ether may act as a perfect fluid for large and com- 

 paratively slow displacements, those occurring in the 

 propagation of light may be so small and so rapid that 

 for them the ether behaves like an elastic soUd. 



The ether must be supposed to freely pervade aU 

 material bodies. As all bodies are compressible, their 

 constituent molecules cannot be in contact, and the ether 

 may be regarded as surrounding and bathing them, the 

 molecules floating as it were in an ocean of ether. But 

 the ether is affected by the presence of the molecules of 

 matter, and this is shown by the bending or refraction of 

 rays of light when they enter transparent substances. 

 This refraction is accounted for in the undulatory theory 

 by the difference of the velocity of the light-waves through 

 a vacuum and through ponderable matter. The presence 

 of the particles of matter causes the speed of transmission 

 of light to be less. At first sight it appears difiicult to 

 believe that a dense solid substance like glass should have 

 its molecules so far apart as to aUow of a free penetration 

 of the ether amongst them, and that this contained ether 

 should also be free to vibrate and thus transmit the 

 periodic disturbances producing light. But we know that 

 a magnet can act through a plate of glass and attract a 

 mass of iron on the other side, and this magnetic influence 

 must be conveyed through some medium from the magnet 

 to the iron. Neither the magnetic influence nor the 

 ethereal medium is directly observable by our senses, but 

 the existence of both is inferred by our intellect from the 

 effects produced. 



From the fact that ether is capable of transmitting with 

 a finite velocity the vibrations which convey light, it 

 would seem to follow that it must be endowed with inertia, 

 or some property answering to mass in ordinary matter. 

 As these vibrations are transverse, it must also possess a 

 quasi-rigidity or an elasticity analogous to that by which 

 a solid body resists a force tending to change its shape, or 

 opposes the gliding of its particles over one another. It 

 may be continuous ; at any rate, if it has a molecular 

 structure it must be different from that of gases, which 

 cannot transmit transverse vibrations. If it be supposed 

 to consist of molecules at a distance from each other, the 

 same difliculty as to action at a distance between these 

 separated particles would occur, for action at a distance 

 across empty space is not more easy to understand when 

 the distance is extremely small than it is when the 

 distances are those we have to deal with in astronomy. 



