74 SPECIAL RESULTS IN THE URANOLOGICAL 



A physicist distinguished for his knowledge as well as for 

 his great delicacy in experimenting, M. Fizeau, lias succeeded 

 in executing a terrestrial measurement of the velocity of 

 light, by means of an ingeniously devised apparatus, in which 

 the artificial star-like light of oxygen and hydrogen is returned 

 to the point from whence it came, by a mirror placed at a 

 distance of 8633 metres (28324 English feet), between 

 Suresne and La Butte Montmartre. A disc, furnished with 

 720 teeth, which made L2'6 revolutions in a second, alter- 

 nately stopped the ray of light, and allowed it to pass freely 

 between the teeth of the limb. Prom the indications of a 

 counter (compteur) it was inferred, that the artificial light 

 traversed 17266 metres (56648 Eng. feet), or twice the 

 distance between the stations, in 1 6 ^ 00 of a second of time ; 

 whence there results a velocity of 167528 geographical miles 

 in a second. ( 142 ) This result comes nearest to that of 

 Delambre derived from Jupiter's satellites, which is 167976 

 geographical miles in a second. 



Direct observations, and ingenious considerations on the 

 absence of any alteration of colour during the change of light 

 of variable stars, (a subject to which I shall presently re- 

 turn), have led Arago to the conclusion that, (in the language 

 of the undulatory theory), rays of light which have different 

 colours, and therefore very different lengths and rapidities of 

 transverse vibration, move through space with equal velo- 

 cities ; but that in the interior of the different bodies through 

 which the coloured rays pass, their rates of propagation and 

 their refractions are different. ( 143 ) Arago' s observations 

 have shown that in the prism the refraction is not altered 

 by the relation which the velocity of light bears to that of 

 the Earth's motion. All the measurements accord in the 



