234 ON LIGHT. 



to measure the velocity of electricity. Without figure?, 

 and without much more verbal detail than would be 

 compatible with our limits, it would be impossible to 

 give a clear conception of the conduct of this delicate 

 and refined experiment. Suffice it to state, as its ulti- 

 mate result, a velocity of 185,172 miles per second.* 

 As there are other and independent reasons for believ- 

 ing that the sun's distance has been over-rated by about 

 one-thirtieth in our estimate of 12,000 diameters of the: 

 earth, and that, in consequence, the velocity of light 

 deduced from the phenomenon of aberration ought to 

 be diminished in the same proportion (which would 

 reduce it to 186,300 miles per second), we are autho- 

 rized to conclude that in estimating this velocity at 

 186,000 miles we are within a thousand miles of the 

 truth. 



(17.) The form of experiment proposed and executed 

 by M. Foucault has this great advantage over the other 

 that it can be carried out within much smaller limits 

 of distance. A few yards of travel suffices for the deter- 

 mination of this enormous speed. And this makes it 

 possible to compare the velocity of light in its passage 

 through air and water, and other transparent liquids 

 with this remarkable result, that the rate is found to be 

 slower in the denser medium ; a result of the utmost 

 importance, as we shall presently see, as a crucial fact m 

 deciding between the claims of the two great rival 

 theories of light to be received as valid. 



* 298 millions of metres. See Comptes Rendus df F Institut^ 

 Sept. 22, 1860. 



