Page:Popular Science Monthly Volume 2.djvu/224

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THAT the expectation of pleasure and profit from the course of lectures which Prof. Tyndall has prepared for delivery in this country is not likely to be disappointed, will appear from the following careful analysis of his theme, Light and Heat, as he has arranged it for six nights:

He begins in a prefatory way, and dwells upon the introduction of the experimental method into Science—speaks of the ardor of investigators and of their rewards. He seeks to show that most of them wrought for the sake of knowledge, and with no practical end in view, though their discoveries travelled to the most astonishing practical applications. After dwelling on the importance of original inquiry, he takes up the real subject of the lectures. The instruments are explained, and the principles upon which they depend. He points out the proximate cause and action of the electric light which is to be used in the illustrations. The laws of reflection are demonstrated, and one or two striking practical applications adduced. Then he goes on to refraction. These elementary subjects are really touched upon in order to enable him, in a subsequent lecture, to reveal the workings of Newton's mind when he theorized upon the subject of light. Refraction is followed by an inquiry into the constitution of light, its analysis and synthesis. This occupies the first lecture.

In the second lecture the demonstrated constitution of light is applied to the doctrine of colors. He goes very thoroughly and plainly into this matter, making perfectly evident the causes on which ordinary colors depend; winding up by the experimental proof that yellow and blue light, when mixed together, produce white and not green. Having exhausted the ordinary spectrum, he describes the difference between the emissions from solids and their vapors. Metallic vapors are produced and shown with their characteristic colors. Their light is then analyzed, and it is shown to be distinctive of the substance from which it comes. Spectrum analysis is dwelt upon, and copiously illustrated.

In his third lecture, Tyndall deals with solar light, dwelling upon the distinction between the bright lines of the metallic vapors, and the dark lines of Frauenhofer. The reciprocity of radiation and absorption is demonstrated, and it is shown experimentally that an incandescent vapor absorbs the light which it emits. This leads up to the theory of the physical constitution of the sun. Then he goes on to show the extension of the spectrum beyond its visible range, performing with quartz prisms and lenses Stokes's experiments on Fluorescence, and the rendering visible of invisible rays. Then the other side of the spectrum is handled; its extension as heat beyond the limits of light is