58 The Velocity of Light. 



then, our conceptions in considering the velocity of light will be 

 materially assisted ; and we shall also be able to draw more perfect 

 analogies from sound, which is propagated through air in a some- 

 what similar manner. To measure the speed of an object, say a 

 ship, a simple method would be to take any known distance and see 

 how long the object takes to traverse that distance. Supposing, for 

 instance, the known distance was a hundred yards, and it was found 

 that it took ten seconds to perform that distance; its speed or velo- 

 city is measured by determining the distance passed over in one 

 second, that is by dividing the distance by the time, which would in 

 the case before us make the speed to be ten yards per second. And 

 similarly, to find out the velocity of sound, a method would be to 

 stand at a known distance from a person who struck a piece of iron 

 with a hammer, and to observe how much after the blow was seen to 

 descend, the sound of it was heard, and then our former rule of 

 dividing the distance by the time would have to be applied. It 

 might naturally be supposed the velocity of light could be found 

 out in a somewhat similar manner. Thus it might be theoretically 

 found out in the following way. Let two men, A and B be 

 stationed at a known distance from each other ; and let each of them 

 possess a lamp and a screen, and let them both put their screens 

 before their lamps. Let A first remove his screen from before 

 his lamp, and let B, as soon as he sees A's light, also remove 

 his screen ; then, if A can find out the time that has elapsed 

 between the removal of his screen and the moment he saw B's light, 

 he would have found out how long light took to travel double the 

 distance between A and B. This would be a theoretical way of 

 discovering the velocity of light. Operating in this way, however, 

 whatever the distance between A and B might be, on the earth, the 

 time taken by light to traverse that distance would be quite inap- 

 preciable by all ordinary methods of observation. Experiments 

 of this nature on the velocity of light led people for a long time to 

 believe that it travelled instantaneously ; and though this is now 

 known not to be true, yet the speed of light is so great that it would 

 traverse the circumference of the earth T^ times in a second. The 

 experimental mode of determining the velocity of light by means of 

 two lamps was actually attempted by Galileo, but without any 

 result. For a long time, therefore, owing to the enormous velocity 

 of light, people had no accurate notion of its speed, and, as I have 

 said, it was usually regarded as instantaneous. To Roemer, a 

 Dutch astronomer, who was brought to France by Picard, in 1672, 

 is due the honour of having been the first to prove that light is not 

 transmitted instantaneously through space, and of having dis- 

 covered approximately, at least, the velocity of light. This im- 

 portant discovery was made by a careful examination of the 



