212 THE POPULAR SCIENCE MONTHLY 



PROFESSOR TYNDALL'S TOPICS. 



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 he disappointed, will apj:>ear from the follow- 

 ing careful analysis of his theme, Light and Heat, as he has arranged 

 it for six nights : 



He hegins in a prefatory way, and dwells upon the introduction of 

 the experimental method into Science speaks of the ardor of inves- 

 tigators 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 ap- 

 plications. After dwelling on the importance of original inquiry, he 

 takes up the real subject of the lectures. The instruments are ex- 

 plained, and the principles upon which they depend. He points out 

 the proximate cause and action of the electric light which is to he 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 or- 

 der to enable him, in a subsequent lecture, to reveal the workings of 

 Newton's mind when he theorized upon the subject of light. Refrac- 

 tion 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 ap- 

 plied 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 ichite and not green. Hav- 

 ing exhausted the ordinary spectrum, he describes the difference be- 

 tween 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 illus- 

 trated. 



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 spec- 

 trum is handled ; its extension as heat beyond the limits of light is 



