OF ARTS AND SCIENCES: APRIL 14, 1863. 117 



portional to the indices of refraction inversely, which in the case pre- 

 sented are as 1 to .000294. This theoretical difference of velocities 

 is less than tj^\j^ of the whole, or less than 70 miles. 



Compare with these conclusions of Astronomy two experimental 

 results on the same subject. Although Wheatstone's experiment on 

 the velocity of electricity, published in 1834, suggested the possibility 

 of measuring, in a similar way, other great velocities, I shall consider 

 first a contrivance of Fizeau, equally applicable to light and to elec- 

 tricity. If a wheel finely cut into teeth on its circumference is put in 

 rapid rotation, a ray of light which escapes between two consecutive 

 teeth will, after being reflected perpendicularly by a mirror, return to 

 strike the wheel at a different point, and either be intercepted by a 

 tooth, or admitted at another interstice. Suppose the velocity of the 

 wheel just sufficient to bring the adjacent tooth to the position whence 

 the ray first started, in the time which the light occupies in going to 

 the mirror and returning. In this time the wheel has moved over an 

 angle found by dividing 360° by twice the number of teeth which the 

 wheel contains. Therefore the time taken by light in going over a 

 line equal to twice the distance of the mirror is that portion of a second 

 found by dividing unity by the product of the number of turns the 

 wheel makes in a second, multiplied by double the number of teeth on 

 the wheel ; the velocity of the wheel being first made the smallest 

 which will cause it to intercept the light. Such an experiment was 

 made in 1849, by Fizeau, the rotating wheel being placed in a tower 

 at Suresne, near Paris, and the mirror upon a hill (Montmartre) at 

 the distance of 8633 metres. As the wheel contained 720 teeth, and 

 the slowest velocity which produced obscui'ation was 12.6 turns a 

 second, it appeared that light required x^ii"?^ ^^ ^ second to go 8633 

 metres and return. Hence the velocity was 313,274,304 metres, or 

 194667 miles a second. The French Academy thought so favorably 

 of this attempt that they referred the subject to a scientific commission 

 consisting of Biot, Arago, Pouillet, and Regnault, with authority to 

 procure a grand machine for repeating the experiment. 



When Arago advocated the claims of Wheatstone to the vacant 

 place of corresponding member of the French Academy in the section 

 of Physics, it was objected that Wheatstone had only made a single 

 experiment, without having discovered a principle. Arago engaged to 

 prove that the candidate had introduced a fertile method of experi- 

 mentation, which would be felt in other sciences as well as electricity. 



