FIG. 226. PROFESSOR TYNDALL. 



CHAPTER XIX. 



PHYSICS OF THE NINETEENTH CENTURY (continued] \ 

 OPTICS, RADIATION, HEAT, AND SOUND. 



THE astronomical means of estimating the velocity of light have 

 already been described, and the amazing amount of that velocity 

 has been mentioned. Not long ago the direct measurement of the 

 speed of light would have been supposed the most impossible of 

 accomplishment. Yet about 1850 the problem was solved in different 

 ways by two eminent French savans. The experiments first announced 

 were those of M. Fizeau, and the principle of his apparatus may be 

 understood from the diagram Fig. 2&fi A beam from a source of 

 light A impinges on a plate of unsilvered glass B so as to be reflected 

 along the axis of a horizontal tube, the extremity of which is provided 

 with a convex lens not shown in the diagram. The lens is so placed 

 that the beam emerges with parallel rays, and at a station several miles 



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