APPENDIX C.

THE following passage from Dr. Whewell's History of the Inductive Sciences' contains so elegant an example of the logical process by which the great generalizations in physical science are established, that, with a view to illustrate some occasional references to the line of reasoning, pursued in physical investigations which occur in the text, I am induced to extract it.

"When we look at the history of the emission-theory of light, we see exactly what we may consider as the natural course of things in the career of a false theory. Such a theory may, to a certain extent, explain the phenomena which it was at first contrived to meet; but every new class of facts requires a new supposition,-an addition to the machinery and as observation goes on, these incoherent appendages accumulate, till they overwhelm and upset the original frame-work. Such was the history of the hypothesis of solid epicycles; such has been the history of the hypothesis of the material emission of light. In its simple form, it explained reflection and refraction; but the colours of thin plates added to it the hypothesis of fits of easy transmission and reflection; the phenomena of diffraction further invested the particles with complex hypothetical laws of attraction and repulsion; polarization gave them sides; double refraction subjected them to peculiar forces emanating from the axes of crystals; finally dipolarization loaded them with the complex and unconnected contrivance of movable polarization; and even when all this had been assumed, additional mechanism was wanting. There is here no unexpected success, no happy coincidence, no convergence of principles from remote quarters the philosopher builds the machine, but its parts do not fit; they hold together only while he presses them: this is not the character of truth.

"In the undulatory theory, on the other hand, all tends to unity and simplicity. We explain reflection and refraction by undulations; when we come to thin plates, the requisite 'fits' are already involved in our fundamental hypothesis, for they are the length of an undulation: the phenomena of diffraction also require such intervals; and the intervals thus required agree exactly with the others in magnitude, so that no new

property is needed. Polarization for a moment checks us; but not long ; for the direction of our vibrations is hitherto arbitrary ;-we allow polarization to decide it. Having done this for the sake of polarization, we find that it also answers an entirely different purpose, that of giving the law of double refraction. Truth may give rise to such a coincidence; falsehood cannot. But the phenomena became more numerous, more various, more strange ;-no matter the theory is equal to them all. It makes not a single new physical hypothesis; but out of its original stock of principles it educes the counterpart of all that observation shows. It accounts for, explains, simplifies, the most entangled cases; corrects known laws and facts; predicts and discloses unknown ones; becomes the guide of its former teacher, observation; and enlightened by mechanical conceptions, acquires an insight which pierces through shape and colour to force and cause."-Vol. ii. p. 464–6.

Such has been the process by which the great inductions in physical investigation have been established. In economic inquiry (as I have shown in my third lecture) this circuitous method is unnecessary, the ultimate facts and assumptions being susceptible of direct proof.

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