Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: LECTURE II. ORIGIN OF PHYSICAL THEORIESSCOPE OF THE IMAGINATIONNEWTON 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-WAVESANALOGIES OF SOUND AND LIGHT ILLUSTRATIONS OF WAVE-MOTIONINTERFERENCE 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 THEORYBOYLE AND HOOKB- -THE COLOURS OF THIN PLATESTHE SOAP-BUBBLENEWTON's RINGS THEORY OF ' FITS ' ITS EXPLANATION OF THE RINGS OVERTHROW OF THE THEORYDIFFRACTION OF LIGHT COLOURS PRODUCED BY DIFFRACTIONCOLOURS 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 the senses, using his hands, eyes, and ears in his experiments : but such a question as that now before us...