C. GENERAL PHYSICS. 147 



ous colors. The presence of oxygen is necessary, according 

 to Wiesner, in order that the rays of light may effect the de- 

 composition of chlorophyll, and this author has made an in- 

 teresting investigation into the action of different portions 

 of the spectrum. He concludes that all the chemical changes 

 caused in chlorophyll cells by the rays of light (namely, 

 their development, decomposition, and assimilation of other 

 substances) take place most rapidly in the brightest portion 

 of the spectrum ; and that, while all portions of the visible 

 spectrum have the power of inducing these changes, the 

 mechanical effects of light upon plants are to be especially 

 ascribed to the rays of high refrangibility. Pogyendorff 

 Anncden, CLIL, 503. 



EXPERIMENTS OX THE VELOCITY OF LIGHT. 



While astronomy has been busy with the problem of the 

 sun's distance, physics has contributed an independent solu- 

 tion to this question. M. Cornu, of the French Academy of 

 Sciences, has repeated the celebrated experiments on the ve- 

 locity of light which were proposed by Foucault and Fizeau 

 some twenty-five years ago. He has himself improved the 

 method of Fizeau, and his experiments were exhaustively 

 conducted and have been perfectly successful. 



M. Cornu chose for his two stations the Observatory of 

 Paris and the tower of Montlhery, whose distance apart 

 (about fifteen miles) is very precisely known. The beam of 

 light passing from the observatory fell upon a toothed wheel 

 revolving no less than 1600 times per second; a portion of 

 the beam, escaping through the interval between two teeth, 

 passed on to a reflector at Montlhery, and returned thence 

 to the revolving wheel. If the rotation of the wheel during: 

 the time required for the light to travel to and fro over 

 twice the distance between the two stations interposes a 

 tooth in the path of the returning ray, an extinction of the 

 luminous impression occurs, and from the known velocity of 

 rotation of the wheel, and from the known distance of the 

 stations, the velocity of light can be had. By revolving the 

 wheel at different rates this extinction can be made to oc- 

 cur at the fourth, fifth, sixth, etc., tooth. A great accordance 

 characterizes Cornu's results, and high importance attaches 

 to this delicate research. All the observations were made at 



