356 INDUCTION. T 



supposition can not be looked upon as more than a conjecture ; the exist- 

 ence of the ether still I'ests on the possibility of deducing from its as- 

 sumed laws a considerable number of actual phenomena ; and this evidence 

 I can not regard as conclusive, because we can not have, in the case of such 

 an hypothesis, the assurance that if the hypothesis be false it must lead to 

 results at variance with the true facts. 



Accordingly, most thinkers of any degree of sobriety allow that an hy- 

 pothesis of this kind is not to be received as probably true because it ac- 

 counts for all the known phenomena; since this is a condition sometimes 

 fulfilled tolerably well by two conflicting hypotheses ; while there are prob- 

 ably many others which are equally possible, but which, for want of any 

 thing analogous in our experience, our minds are unfitted to conceive. But 

 it seems to be thought that an hypothesis of the sort in question is entitled 

 to a more favorable reception, if, besides accounting for all the facts previ- 

 ously known, it has led to the anticipation and prediction of others which 

 experience afterward verified ; as the undulatory theoi-y of light led to the 

 prediction, subsequently realized by experiment, that two luminous rays 

 might meet each other in such a manner as to produce darkness. Such 

 predictions and their fulfillment are, indeed, well calculated to impress the 

 uninformed, whose faith in science rests solely on similar coincidences be- 

 tw^een its prophecies and what comes to pass. But it is strange that any 

 considerable stress should be laid upon such a coincidence by persons of 

 scientific attainments. If the laws of the propagation of light accord with 

 those of the vibrations of an elastic fluid in as many respects as is necessa- 

 ry to make the hypothesis afford a correct expression of all or most of the 

 phenomena known at the time, it is nothing strange that they should ac- 

 cord with each other in one respect more. Though twenty such coinci- 

 dences should occur, they would not pi'ove the reality of the undulatory 

 ether ; it would not follow that the phenomena of light were results of the 

 laws of elastic fluids, but at most that they are governed by laws partially 

 identical with these ; which, we may observe, is already certain, from the 

 fact that the hypothesis in question could be for a moment tenable.* 

 Cases may be cited, even in our imperfect acquaintance with nature, where 

 agencies that we have good reason to consider as radically distinct produce 

 their effects, or some of their effects, according to laws which are identical. 

 The law, for example, of the inverse square of the distance, is the measure 

 of the intensity not only of gravitation, but (it is believed) of illumination, 

 and of heat diffused from a centre. Yet no one looks upon this identity 

 as proving similarity in the mechanism by which the three kinds of phe- 

 nomena are produced. 



According to Dr. Whewell, the coincidence of results predicted from an 

 hypothesis with facts afterward observed, amounts to a conclusive proof 

 of the truth of the theory. " If I copy a long series of letters, of which 

 the last half-dozen are concealed, and if I guess these aright, as is found 

 to be the case when they are afterward uncovered, this must be because I 

 have made out the import of the inscription. To say that because I have 



* What has most contributed to accredit the hypothesis of a physical medium for the con- 

 veyance of light, is the certain fact that light travels (which can not be proved of gravita- 

 tion) ; that its communication is not instantaneous, but requires time ; and that it is intercepted 

 (which gravitation is not) by inten'ening objects. These are analogies between its phenome- 

 na and those of the mechanical motion of a solid or fluid substance. But we are not entitled 

 to assume that mechanical motion is the only power in nature capable of exhibiting those at- 

 tributes. 



