Jan. 1, 1886.] 



♦ KNO\VLEDGE ♦ 



101 



properties whicli liiive beeu necessarily assigned to the 

 ether of space. 



* * * 



The idea that because we cannot actually analyse phj'si- 

 c^lly or chemically some particular entity (whose very 

 nature may place it outside such analysis) we therefore 

 must not admit its existence, is altogether unscientific. (In 

 Mr. Williams's case there is I know the belief in highly 

 attenuated air to conflict with the acceptance of ether — 

 an excuse though not really a I'eason : but I do not care 

 to remark upon his attenuated air theory beyond saying 

 of that air that it is too thin.) 



Consider the following example : — 



There is a bowl of glass which we know to be such 

 because we can study its outer surface ; but we cannot 

 examine its interior, nor test its weight, because it is 

 fastened by some rigid support. We move so that a light 

 is seen through the globe, and then moving the eye 

 sideways find the light draw near the edge in the same 

 direction, but never actually reach the edge : we are now 

 justified in assuming that the globe is only a shell of glass 

 with air or some other medium much rarer than glass 

 inside. This assumjation is absolutely sound, in this 

 general form. More careful observation may enable the 

 mathematician to say just how thick the glass shell is, 

 and just what refractive power the medium inside 

 possesses. Supposing this refractive power to be that 

 which air at the average sesr-level pressure possesse.s, the 

 observer might infer with considerable probability that 

 there was air within the shell. But on this point he 

 could not be certain ; for other gases than those in air 

 might at certain pressures have the same refractive 

 power. 



* * * 



Or " put it my juvenile friends," as the unctuous 

 Chadband hath it, that our observer on moving his eye 

 to the right found the light pass off to the left, until, 

 moving continuously but not uniformly to the left (as 

 his eye moved uniformly to the right) it came right up 

 to the very outline of the disc presented by the bowl. 

 Then he would be justified in inferring that the bowl 

 was not a mere .shell of glass, and that, if not a globe of 

 uniform glass throughout, its interior varied continuously 

 in density, not by any sudden or abrupt change as from 

 water to glass. If he found the final direction of the 

 light when seen on the very verge of the disc to corre- 

 spond to the direction due to the refractive power of 

 glass of the same quality as the tested outside of the 

 globe, he might infer with considerable probability that 

 the globe was throughout of the same material. But 

 this would not be absolutely certain. 



* * * 



If, however, he found that while his eye moved 

 uniformly to the right the light moved continuously up 

 to a certain distance from the edge of the disc presented 

 by the bowl, then became indistinct for a moment, and 

 presently was seen nearer the edge and eventually came 

 right up to it, — then the observer would be justified iu 

 assuming that there was a shell of glass, enclosing a 

 medium of somewhat smaller refractive power, as water, 

 for example. He might be able to determine observa- 

 tionally the precise thickness of the glass shell ; and 

 further, assuming that the medium within was really 

 water, he might, by a series of observations, test this 

 assumption. If ho found that the medium certainly was 

 not water, and had not the refractive power of any sub- 

 stance he had tested, it would still be a demonstrated 



fact that there was some medium of such and such 

 refractive power within the shell. 



* * * 



In like manner it is a demonstrated fact that there is 

 a luminiferous medium, possessing such and such pro- 

 perties, — but its actual nature not wholly determined or 

 probably determinable. To dogmatise as to the undeter- 

 mined properties of this ether would be an unscientific 

 proceeding ; but to doubt the demonstrated properties is 

 to indicate want of power to appreciate the investigations 

 (certainly rather recondite in parts) by which these pro- 

 perties have been ascertained. Precisely so, to take a 

 much easier subject, an imrfgined student of our illustra- 

 tive bowl would not act as a scientific man should, if he 

 assumed in the third case that the enclosed medium, as 

 yet undetermined, had such and such chemical pro- 

 perties ; but for him to doubt its refractive power would 

 simply indicate his inability to understand the laws 

 according to which rays of light pass with refraction, 

 from all but tangential contact into a spherical glass shell, 

 full of a medium nearly as refractive as itself, and thence 

 tangential ly out of it. 



* * * 



Of course inability to understand such a simple 

 experiment as this (the reader can readily try it with a 

 globular decanter — or a tumbler will do — first empty, 

 then full), and inability to follow such difficult mathe- 

 matical calculations and such delicate physical experi- 

 ments as have been used to establish the undulatory 

 theoi-y of light and to demonstrate the general qualities 

 of the luminiferous ether, are very different matters : 

 but the principle is the same. 



* * * 



The advocates of the " poor old theory of phlogiston " 

 opposed, through precisely such unreadiness to admit the 

 force of evidence, the introduction of the theory that 

 heat is not a substance but a mode of motion. So did 

 the advocates of the emission of some actual matter in 

 rays oppose the undulatory theory of light. But their 

 objections remain now merely " to point a moral or adorn 

 a tale." 



* * * 



On the other hand, I do not feel quite so clear that my 

 friend Mr. Clodd has been well advised in giving new 

 definitions and altered meanings to the much misused 

 words Force and Energy. Of cour.'e, as he does define, 

 very precisely, the sense in which lie |U'oposes to use the 

 words, the reader of his thoughtful essay need not care 

 to consider whether the words should be so understood 

 generally or not. In the well-known story of the under- 

 graduate under examination who had in a descriptive 

 answer written throughout of seven somethings (I forget 

 what) where, as he remembered towards the close he 

 should have said five, that ingenious examinee to save 

 the trouble of going through his reply and correcting 

 every " seven " into a " five," made all right by prefixing 

 this note, " In the throe following pages let the symbol 

 7 represent 5, and let the word " seven " represent the 

 word " five." So, but with intention aforethought, Mr. 

 Clodd, at Mr. Allen's suggestion, gives meaning to his re- 

 marks abiiut Force and Energy by explaining at the outset 

 that iu what follows the word " Force " stands for some- 

 thing akin to what is ordinarily called " Attraction," and 

 the word Energy for " Repulsion." All the reader has, 

 then, to consider is whether the statements so understood 

 are correct. It is to be feared, perhaps, that the con- 

 fusion which has appeared in certain important philo- 



