Notes of a Course of Nine Lectures on Light: Delivered at the Royal Institution of Great Britain, April 8-June 3, 1869

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Longmans, Green, 1870 - 74 pages
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Page 10 - ... image of L. Moreover it will be found from the figure by direct measurement that LL' is perpendicular to the mirror and that L'A is equal to LA. Thus the image of a point formed by a plane mirror is virtual and its position is obtained by drawing a normal from the point to the mirror, and producing it as far behind the mirror as the point is in front of it. This result may be verified in the following way. EXPERIMENT (7). To verify the position of the image of a voint formed by a plane mirror.
Page 31 - The justification of a theory consists in its exclusive competence to account for phenomena. On such a basis the Wave Theory, or the Undulatory Theory of Light, now rests, and every day's experience only makes its foundations more secure. . . . This substance is called the luminiferous ether.
Page 36 - It is worth while to mark how this experiment illustrates the fact that, however intense a luminous beam may be, it remains invisible unless it has something to shine upon. Space, though traversed by the rays from all suns and all stars, is itself unseen. Not even the ether which fills space, and whose motions are the light of the universe, is itself visible.
Page 32 - In the case of sound, the vibration of the airparticles are executed in the direction in which the sound travels. They are therefore called longitudinal vibrations. In the case of light, on the contrary, the vibrations are transversal; that is...
Page 73 - Grant's lucid account of all that it explains. On a precisely similar basis rests the undulatory theory of light; only that the phenomena which it explains are far more varied and complex than the phenomena of gravitation. You regard, and justly so, the discovery of Neptune as a triumph of theory. Guided by it, Adams and Leverrier calculated the position of a planetary mass competent to produce the disturbances of Uranus.
Page 19 - ... to light. It is the frequency of the reflections at the limiting surfaces of air and water that renders foam opaque. The blackest clouds owe their gloom to this repeated reflection, which diminishes their transmitted light, hence also their whiteness by reflected light. To a similar cause is due the whiteness and imperviousness of common salt, and of transparent bodies generally when crushed to powder. The individual particles transmit light freely; but the reflections at their surfaces are so...
Page 35 - All these waves enter the eye in a second. In the same interval 699,000,000,000,000 waves of violet light enter the eye. At this prodigious rate is the retina hit by the waves of light.

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