and the Sun's Distance. . 527 



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 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 

 feeth, and the slowest velocity which produced obscuration was 

 12*6 turns a second, it appeared that light required tfitt of a 

 second to go 8633 metres and return. Hence its velocity was 

 313,274,304 metres, or 194,667 miles a second. The French 

 Academy thought so favourably of this attempt that they re- 

 ferred 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 experimentation which would be 

 felt in other sciences as well as electricity. For example, the 

 corpuscular theory of light requires that the velocities of light 

 in different media should vary directly as the indices of refrac- 

 tion, whereas the undulatory theory inverts this ratio. Arago 

 prepared for the trial by experiments on rapid rotation, the 

 mechanical difficulties to be overcome, and the comparative 

 advantage of slower rotations assisted by several reflexions in 

 place of a single mirror turning with its maximum speed. 

 Aided by the refined skill of Breguet, he realized velocities in the 

 mirror of 1000 turns a second, and of the axis detached from the 

 mirror, of even 8000 turns. In the meanwhile his eyesight began 

 to fail him, and younger physicists entered into the fruit of his 

 labours. After Foucault and Fizeau, by separate efforts, had 

 decided the question in relation to the velocities of light in air 

 and in water, in favour of the undulatory theory, and thus con- 

 firmed a conclusion which Arago reached by diffraction in 1838, 



