;o6 



NA TURI-: 



[July 27, 189: 



and other festivities, at «hich llie foicign members were the 

 lions of the occafii n ; indeed, international couifesy reigned 

 throughout the pro<ecdirgs. This was carried so lar in one 

 section that hardly anything hut French was spoken, those who 

 wished to take part in the discussions receiving but little en- 

 couragement from the chair unless they addressed the meeting 

 in the French tongue— or, lather, in French words. This was 

 satislactory to the majcriiy, so far as the remarks of foreign 

 members were concerned ; but when the language was exotic in 

 its character to follow was sometimes laboiious. This week a 

 series of excursions are being made to some of the chief ports 

 of the United Kingdom. 



THE LUMINIFEROUS ETHER. 

 AT the anniversary meeting of the Victoria InUitute on June 

 29, Sir G. G. Stokes delivered his presidential address. 

 After a few introductory remarks on the functions of the 

 Institute, he said : — " I intend to bring before you to night a 

 subject which the study of light has caused me to think a good 

 deal about ; 1 refer to the nature and properties of the so-called 

 luminiferous ether. This subject is, in one respect, specially 

 fascinating, scientifically considered. It lies, we may say, in an 

 especial manner on the borderland between what is known and 

 what is unknown. In the study of it it is quite conceivable 

 that great discoveries may lie made, and, in fact, great discoveries 

 have already been made, and I may say even quite recently, and 

 we do not at present know how much additional light on the 

 system of Nature may be in store for the men of Science ; possibly 

 even in the near future, possiiily not until many generations have 

 passed away. I will assume, as what is familiarly known to you 

 all, and what is well established liy methods into which I will 

 not enter, that the heavenly bodies are at an immense distance 

 from our earth. More especially is this the case with the fixed 

 stars. Their distance is so enormous that even when we take as 

 a base Hue, so to speak, the diameter of the earth's orbit, which 

 we know to be about 184 millions of miles, the apparent dis- 

 placement of the stars due to parallax is so minute as almost to 

 elude our investigation. Nevertheless that distance is more or 

 less accurately determined in the case of a few of the fixed stars. 

 But the vast majority, as we have every reason to believe, are 

 at such an enormous distance that even this method fails with 

 them." 



"To give a conception of the immense distance of the fixed 

 stars, I will assume as known that light travels at the rate of 

 about 186,000 miles in one second, a rate which would carry it 

 nearly eight times round and round the earth in that time ; 

 and yet if we take the star which, so far as we know, is our 

 nearest neighbour, it would take three or four years for light 

 from that starto reach the earth. Now as we see the fixed stars 

 there must be some link of connection between us and them in 

 order that we should be able to perceive them. Probably all 

 of you know that two theories have been put forward as to the 

 nature of light, as to the nature accordingly of that connection 

 of which I have spoken. According to one idea, light is a sub- 

 stance darted forth from the luminous body with an amazing 

 velocity ; according to the other, it consists in a change of state 

 taking place, propagated through a medium, as it is called, in- 

 tervening between the body from which the light proceeds and 

 the eye of the observer. For a considerable time the first of 

 these theories was that chiefly adopted by scientific men. It 

 was that, as you know, which Newton himself adopted ; and 

 probably the prestige of his name had much to do with the 

 favourable reception which for a long time it received. But 

 more recent researches have so completely established the truth 

 of the other view, and refuted the old doctrine of emissions, 

 that it is now universally held by scientific men that light con- 

 sists in an undulatory movement propagated in a medium 

 existing in all the space through which light is capable of pa-s- 

 ing." 



" This necessity for filling all space, or at least, such an in- 

 conceivably great extent of space, with a medium, the office of 

 which, so far as was known in the first instance, was simply 

 that of propagating light, was an obstacle for a time to the recep- 

 tion by the minds of some of the theory of undulations. Men 

 had been in the habit of regarding the inter-planetary and inter- 

 s'.eliar Sjiaceasa vaciiuiii, and it seemed too great an assumption 

 to fill all this supposed vacuous space with some kind of medium 

 for the sole purpose of transmitting light. Notwithstanding, 



NO. 1 2-59, VOL. 4S] 



even long ago strong opinions were entertained to the effect that 

 there must be something intervening between the different 

 heavenly bodies. In a letter to lientley, Newton expresses him- 

 self in very strong language to this effect : ' That gravity should 

 be innate, inherent and essential to matter, so that one body 

 may act upon another at a distance through a vacuum, 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 that 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 ac- 

 cording to certain fixed laws ; but whether this agent be 

 materia! or immaterial, I have left to the consideration of my 

 readers.'" 



"What the nature of the connection between the earth and 

 the sun, for example, may be whereby the sun is able to attract 

 the earth and thereby keep it in its orbit— in other words, wliat 

 the cause of gravitation may be — we do not know ; for anything 

 we know to the contrary, it may be connected with this inter- 

 mediate medium or luminiferous ether. There are other offices, 

 we believe, which this luminiferous ether fulfils, to which I shall 

 have occasion to allude presently." 



" In connection with the necessity for filling such vast regions 

 of space with this medium, a curious question naturally presents 

 itself. We cannot conceive of space as other than infinite, but 

 we habitually think of matter as occupying here or there limited 

 portions of space, as, for example, the different heavenly bodies. 

 The intervening space we commonly think of as a vacuum, and 

 it is only the phenomena of light that led us in the first instance 

 to think of it as filled with some kind of material. The question 

 naturally presents itself to the mind — is this ether absolutely in- 

 finite like space? This is a quest ion to which science can give no 

 answer. Though we cannot help thinking of space as infinite, 

 yet when we turn our thoughts to some material existing in space 

 perhaps we more readily think of it as finite than infinite. But 

 if the ether, however vast the portion of space over which it ex- 

 tends, be really limited, we can hardly fail to speculate what 

 there may lie out ide its limits. Space there might be wholly 

 vacuous, or possibly outside altogether this vast system of stars 

 and ether there may be another system subject to the same laws, 

 or subject to different laws, as the case may be, equally vast in 

 extent ; and if there be, then so far as we can gather from such 

 phenomena as are open to our investigation, there can be no 

 communication between that vast portion of space in part of 

 which we live and an ideal system altogether outside the ether 

 of which we have been speaking." 



" But the properties of the ether are no less remarkable than 

 its vast or even possibly limitless extent. Matter of which our 

 senses give us any cognisance is heavy, that is to say, it gravi- 

 tates towards other matter which agrees with it in so far as 

 being accessible to our senses. The question presents itself to 

 the mind, does the ether gravitate towards what we call ponder- 

 able matter? This is a question to which we are not able to 

 give any positive scientific answer. If the ether be in some way 

 or other connected with the cause of gravitation, it would seem 

 more bkely that it itself does not gravitate towards ponderable 

 matter." 



" Again, we have very strong reason for believing that ponder- 

 able matter consists of ultimate molecules. First, that sap- 

 position accords in the simplest way with the laws of crystallo- 

 graphy. Chemical laws afford still stronger confirmation of the 

 hypothesis, through the atomic theory of Dalton, now univers- 

 ally accepted. Comparatively recently, the deduction of the 

 fundamental property of gases from the kinetic theory, as it is 

 called, affords strong additional confirmation of that view of 

 the constitution of matter. Still more recently, the explana- 

 tion which has been afforded by that theory of that most 

 remarkable instrument the radiometer of Crookes has lent 

 further confirmation in the same direction. None of these 

 evidences apply to the ether, and accordingly we are left in 

 doubt whether it too consists of ultimate molecules, or whether 

 on the other hand it is continuous, as we cannot help conceiving 

 space to be." 



" The undulatory theory of light was greatly promoted in the 

 first instance by the known phenomena of sound, and the 

 explanation which they received from the hydrodynamical theory. 

 Accordingly, since sound, as we know, consists of an undulatory 

 movement propagated through the air (or it may le through 

 other media), and depending upon condensation and rarefrac- 

 tion, it was supposed naturally that light was propagated in a 



