THE VELOCITY OF LIGHT. 213 



The ancients had very vague notions about the nature of 

 light. They supposed that something was sent out from 

 the eye which acted the part of a feeler, to inform a person 

 of the position of different objects. This is quite opposed 

 to all our knowledge of physics. The true theory could 

 not fail to gain ground so soon as it was enunciated, viz., 

 that something passes from luminous objects, and either 

 strikes our eyes directly, or may do so after having first 

 been intercepted by other objects. Thus we obtain an idea 

 of the existence of luminous and illuminated objects. 

 What is this something that passes to the nerves of the 

 eye ] This question has been answered by the two theories 

 that were for a long time rivals ; the corpuscular theory 

 supposed that it was matter in a finely divided form that 

 performed this function. The undulatory theory, on the 

 other hand, assumed the exciting cause to be an undulation 

 like the waves of the sea, that are propagated through a 

 medium filling all space, and permeating the mass, at least 

 of all transparent bodies. It is not my purpose tc-day to 

 explain to you the number of proofs that combine to render 

 this theory more than probable, but we shall see how the 

 experiments about to be described settle the question 

 between the corpuscular theory, as enunciated by Newton, 

 and the undulatory theory. This last was first propounded 

 by Huyghens in 1690, and was taught in the following year 

 at St. Andrew's University by James Gregory. But the 

 full expounding of it was done chiefly by Fresnel and 

 Young at the beginning of the present century. 



But let us return to the question of the velocity of light. 

 The first man who gave reasons for believing that light 

 takes a sensible time to pass through space, was the Danish 

 astronomer Roemer. He found at his observatory at 

 Copenhagen that the eclipses of Jupiter's satellites could not 

 otherwise be explained than on the assumption of a uniform 

 revolution of these satellites round the planet. Moreover, 

 he made the curious discovery that this inequality depended 

 on the position of the earth, and that in consequence it 

 could hardly be due to any real inequality of motion of the 

 satellites round the planet. The fact that he noticed was 

 this, that when Jupiter was on the side of the sun away 

 from the earth, the eclipses were later than the predicted 

 time, and when on the same side they were earlier. Now 



