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LECTURE II. ORIGIN OF PHYSICAL THEORIES—SCOPE OF THE IMAGINATION—NEWTON AND THE EMISSION THEORY VERIFICATION OF PHYSICAL THEORIES— THE LUMINIFEROU9 ETHER WAVE-THEORY OF LIGHT THOMAS YOUNG —FHESNEL AND ARAGO CONCEPTION OF WAVE-MOTION INTERFERENCE OF WAVES CONSTITUTION OF SOUND-WAVES—ANALOGIES OF SOUND AND LIGHT ILLUSTRATIONS OF WAVE-MOTION—INTERFERENCE OF SOUND WAVES OPTICAL ILLUSTRATIONS PITCH AND COLOUR LENGTHS OF THE WAVES OP LIGHT AND RATES OF VIBRATION OF THE F.THER- PARTICLES INTERFERENCE OF LIGHT PHENOMENA WHICH FIRST SUGGESTED THE UNDULATORY THEORY—BOYLE AND HOOKB- -THE COLOURS OF THIN PLATES—THE SOAP-BUBBLE—NEWTON's RINGS THEORY OF ' FITS ' ITS EXPLANATION OF THE RINGS OVERTHROW OF THE THEORY—DIFFRACTION OF LIGHT COLOURS PRODUCED BY DIFFRACTION—COLOURS OF MOTHER-OF-PEARL. § I. Origin and Scope of Physical Theories. WE might vary and extend our experiments on Light indefinitely, and they certainly would prove us to possess a wonderful mastery over the phenomena. But the vesture of the agent only would thus be revealed, not the agent itself. The human mind, however, is so constituted and so educated, as regards natural things, that it can never rest satisfied with this outward view of them. Brightness and freshness take possession of the mind when it is crossed by the light of principles, shewing the facts of Nature to be organically connected. Let us, then, inquire what this thing is that we ii. CONCEPTION OF PHYSICAL THEORY. 43 have been generating, reflecting, refracting and analyzing. In doing this, we shall learn that the life of the experimental philosopher is twofold. He lives, in his vocation, a life of thesenses, using his hands, eyes, and ears in his experiments : but such a question as that now before us...
