THE VELOCITY OF LIGHT. 223 



been brought into play during the last few years by the 

 labours of Mr. Huggins by means of spectroscopic observa- 

 tions of stars, he is enabled to measure the velocity of the 

 light of different stars towards or from the solar system, 

 not at right angles to the line of sight, but in the line of 

 sight ; but those researches, the truth of which was con- 

 siderably doubted by German astronomers until lately, 

 have been confirmed by a magnificent new spectroscope 

 lately erected in the Observatory at Greenwich. In order- 

 now to determine the velocity of approach or recession of 

 different stars from or towards the earth, it is necessary to 

 know with considerable exactness the velocity of light, and 

 this is another reason why it is important to have this 

 constant accurately determined. 



Thirdly, and perhaps the most important of all, Professor 

 Clerk Maxwell showed many years ago, in following out a 

 brilliant idea of Faraday, that all electrical and magnetic 

 phenomena are dependent upon the existence of a medium 

 filling space between the electrified and magnetised bodies, 

 and that there are lines of tension along the lines of force, 

 and lines of compression perpendicular to the lines of force. 

 Professor Maxwell, in following out this brilliant idea, 

 and putting it into mathematical language, has been led to 

 the conclusion that the function played by this medium 

 imagined by Faraday is exactly such a function as could 

 be played by the ether, of whose existence we have inde- 

 pendent proof. And moreover there is a certain numerical 

 quantity in electrical measurement which has been measured 

 with great accuracy by Sir "William Thomson and his 

 assistant, a quantity which connects two systems of elec- 

 trical measurements the electro-statical system, and the 

 electro-magnetic system of measurement. This quantity 

 Professor Clerk Maxwell states, ought to be exactly the 

 same as the velocity of light if the ether performs the func- 

 tion of that medium which Faraday conceived, and he has 

 pointed out that this quantity, Y, determined in electric 

 measurement is as nearly as experiment will allow exactly 

 the same as the velocity of light. This enormously im- 

 portant result of Clerk Maxwell's researches gives the 

 determination of the velocity of light and of that constant 

 in the electrical measurement of matter of the very utmost 

 importance, because it is very likely to lead to a more 



