THE WAVE THEORY OF LIGHT AND THE LUMINIFEROUS ETHER
Whewell defends the wave theory of light against the particle theory on the grounds that it is “consilient” (it explains and successfully predicts a range of different optical phenomena, not just ones such as diffraction that were initially used in reviving the Huygensian wave theory at the beginning of the nineteenth century).11 Furthermore, Whewell claims, it is “coherent” (as the theory changes over time to accommodate new phenomena, the revisions “tend to simplicity and harmony” and avoidance of ad hoc hypotheses).
By contrast, Mill argues that Whewells criteria are not sufficient to establish the wave theory, or even to make it probable. He focuses on one central supposition of this theory, viz. that by analogy with water in the case of water waves, and with air in the case of sound, there exists a substance, the ether, that is waving. Mill writes:
At present, however, this supposition cannot be looked upon as more than a conjecture; the existence of the ether still rests on the possibility of deducing from its assumed laws a considerable number of actual phenomena; and this evidence I cannot regard as conclusive, because we cannot 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 hypothesis of this kind is not to be received as probably true because it accounts for all the known phenomena, since this is a condition sometimes fulfilled tolerably well by two conflicting hypotheses, while there are probably many others which are equally possible, but which, for want of anything analogous in our experience, our minds [119] are unfitted to conceive. But it seems to be thought that an hypothesis of the sort in question is entitled to a more favourable reception, if, besides accounting for all the facts previously known, it has led to the anticipation and prediction of others which experience afterwards verified; as the undulatory [wave] theory 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 [destructive interference].
Such predictions and their fulfillment are, indeed, well calculated to impress the uninformed.... If the laws of the propagation of light accord with those of the vibrations of an elastic fluid [the hypothetical ether] in as many respects as is necessary to make the hypothesis a correct expression of all or most of the phenomena known at the time, it is nothing strange that they should accord with each other in one respect more. Though twenty such coincidences should occur, they would not prove the reality of the undulatory ether.[120]The assumption of the existence of light-bearing (“luminiferous”) ether is a central one in the wave theory. What Whewell is saying, and Mill is denying, is that this assumption by itself is not subject to experimental test. Only the entire wave theory is. If that theory as a whole—ether and all—can successfully explain and predict different phenomena and satisfy “coherence,” then the theory as a whole can be regarded as proved or at least highly probable. There is no such thing as proving or showing probable a particular assumption within that theory. What Mill, the particularist, is saying is that you can't regard the theory as a whole as proved or shown to be probable without showing that each assumption within that theory is proved or probable. For Mill, the wave theory is a speculation. Moreover, he is saying, you cannot show that the theory as a whole is proved or probable, and hence not a mere speculation, simply by showing that it satisfies “consilience” and “coherence.”
Mill's particularism, then, leads him to say that in order to prove the wave theory, or to show that it is probable, you need to show that the ether exists. For Mill, in mid-nineteenth century, there is serious scientific work to be done in establishing the wave theory or showing that it is probable: Find the ether. For Whewell, in mid-nineteenth century, there is no such work. The wave theory is established. Indeed, Whewell claims this, even on the basis of “consilience” alone. Citing his two favorite theories, Newton's theory of gravity and the wave theory of light, Whewell writes:
The theory of universal gravitation and of the undulatory theory of light, are, indeed, full of examples of this Consilience of Inductions.... No example can be pointed out, in the whole history of science, so far as I am aware, in which this Consilience of Inductions has given testimony in favour of an hypothesis afterwards discovered to be false.[121]
In short, Mill's particularism and Whewell's holism generate very different conceptions of what the wave theorist can and must do to establish the theory or show it to be probable.
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