6 4 



LECTURES AND ESSA YS 



real than that of the senses, and of which 

 the world of sense itself is the suggestion 

 and, to a great extent, the outcome. 



Far be it from me, however, to wish 

 to fix you immovably in this or in any 

 other theoretic conception. With all 

 our belief of it, it will be well to keep 

 the theory of a luminiferous ether plastic 

 and capable of change. You may, 

 moreover, urge that, although the phe- 

 nomena occur as if the medium existed, 

 the absolute demonstration of its exist- 

 ence is still wanting. Far be it from me 

 to deny to this reasoning such validity 

 as it may fairly claim. Let us endeavour 

 by means of analogy to form a fair 

 estimate of its force. You believe that 

 in society you are surrounded by reason- 

 able beings like yourself. You are, 

 perhaps, as firmly convinced of this as of 

 anything. What is your warrant for this 

 conviction ? Simply and solely this : your 

 fellow-creatures behave as if they were 

 reasonable; the hypothesis, for it is 

 nothing more, accounts for the facts. To 

 take an eminent example : you believe 

 that our President is a reasonable being. 

 Why? There is no known method of 

 superposition by which any one of us 

 can apply himself intellectually to any 

 other, so as to demonstrate coincidence 

 as regards the possession of reason, If, 

 therefore, you hold our President to be 

 reasonable, it is because he behaves as if 

 he were reasonable. As in the case of 

 the ether, beyond the " as if" you can- 

 not go. Nay, I should not wonder if 

 a close comparison of the data on which 

 both inferences rest caused many re- 

 spectable persons to conclude that the 

 ether had the best of it. 



This universal medium, this light-ether 

 as it is called, is the vehicle, not the 

 origin, of wave-motion. It receives and 

 transmits, but it does not create. Whence 

 does it derive the motions it conveys ? 

 For the most part from luminous bodies. 

 By the motion of a luminous body I do 

 not mean its sensible motion, such as 

 the flicker of a candle, or the shooting 

 out of red prominences from the limb 

 of the sua. I mean an intestine motion 



of the atoms or molecules of the lumin- 

 ous body. But here a certain reserve is 

 necessary. Many chemists of the pre- 

 sent day refuse to speak of atoms and 

 molecules as real things. Their caution 

 leads them to stop short of the clear, 

 sharp, mechanically intelligible atomic 

 theory enunciated by Dalton, or any 

 form of that theory, and to make the 

 doctrine of " multiple proportions " their 

 intellectual bourne. I respect the 

 caution, though I think it is here mis- 

 placed. The chemists who recoil from 

 these notions of atoms and molecules 

 accept, without hesitation, the Undula- 

 tory Theory of Light. Like you and me, 

 they one and all believe in an e ther and 

 its light-producing waves. Let us consider 

 what this belief involves. Bring your 

 imaginations once more into play, and 

 figure a series of sound-waves passing 

 through air. Follow them up to their 

 origin, and what do you there find ? A 

 definite, tangible, vibrating body. It may 

 be the vocal chords of a human being, it 

 may be an organ-pipe, or it may be a 

 stretched string. Follow in the same 

 manner a train of ether-waves to their 

 source, remembering at the same time 

 that your ether is matter, dense, elastic, 

 and capable of motions subject to, and 

 determined by, mechanical laws. What 

 then do you expect to find as the source 

 of a series of ether-waves ? Ask your 

 imagination if it will accept a vibrating 

 multiple proportion a numerical ratio 

 in a state of oscillation ? I do not think 

 it will. You cannot crown the edifice 

 with this abstraction. The scientific 

 imagination, which is here authoritative, 

 demands, as the origin and cause of a 

 series of ether-waves, a particle of vibrat- 

 ing matter quite as definite, though it 

 may be excessively minute, as that which 

 gives origin to a musical sound. Such a 

 particle we name an atom or a molecule. 

 I think the intellect, when focussed so as 

 to give definition without penumbral 

 haze, is sure to realise this image at the 

 last. 



With the view of preserving thought 



