202 OBJECTIONS REMOVED. SECT. XXIII. 



whose refrangibility lias more recently been ascertained by M. 

 Rudberg. Thus, in the case of seven rays in each of twenty 

 different substances, solid and fluid, the dispersion of light takes 

 place according to the laws of the undulatory theory : and there 

 can hardly be a doubt that dispersion in all other bodies will be 

 found to follow the same law. It is, however, an express condi- 

 tion of the connexion between the velocity of light and the 

 length of its undulations, that the intervals between the vibrating 

 molecules of the ethereal fluid should bear a sensible relation to 

 the length of an undulation. The coincidence of the computed 

 with the observed refractions shows that this condition is fulfilled 

 within the refracting media ; but the aberration of the fixed stars 

 leads to the inference that it does not hold in the ethereal regions, 

 where the velocities of the rays of all colours are the same. 

 Strong as all that precedes is in favour of the undulatory theory, 

 the relative velocity of light in air and water is the final and 

 decisive proof. By the Newtonian theory the velocity is greater 

 in water than in air, by the undulatory theory it is less ; hence if 

 a comparison could be made it would decide which is the law of 

 nature. The difficulty consisted in comparing the velocity of 

 light passing through a small extent of water with the velocity 

 of light in air, which is 10,000 times greater than the velocity of 

 the earth in its orbit. This delicate and difficult experiment 

 was made by means of an instrument invented by Professor 

 Wheatstone for measuring the velocity of electricity. It consists 

 of a small mirror which revolves in its own plane like a coin 

 spinning on its edge. When it revolves very rapidly the re- 

 flected image of an object changes its place perceptibly in an 

 inconceivably small fraction of a second. The mirrors used in 

 the experiment were made to revolve more than 1000 times in 

 a second, by which means the places of the two images one 

 from light passing through air, and the other from light passing 

 through an equal length of water were found to be such as to 

 prove that the velocity of light in air and in water is as 4 to 3, 

 while by the Newtonian theory it is as 3 to 4. By this final 

 and decisive proof the undulatory theory may from henceforth 

 be regarded as the law of nature. This experiment was accom- 

 plished by M. Fizeau and M. Le'on-Faucault, at the suggestion 

 of M. Arago, whose eyesight did not permit him to undertake it 

 himself. 



