54 CLASSICS OF MODERN SCIENCE 



As soon as we consider, moreover, the enormous speed with which 

 light travels in every direction, and the fact that when rays come from 

 different directions, even from those exactly opposite, they cross 

 without interference, it must be plain that we do not see luminous 

 objects by means of particles transmitted from the objects to us, as a 

 shot or an arrow moves through the air. For surely this would not 

 allow for the two qualities of light just mentioned, particularly the 

 latter (that of speed). Light, then, is transmitted in some other way, 

 a comprehension of which we may get from our knowledge of how 

 sound moves through the air. 



We know that sound is sent out in all directions through the medium 

 of the air, a substance invisible and impalpable, by means of a motion 

 that is communicated successively from one part of the air to the next ; 

 and as this movement has the same speed in all directions, it must 

 form spherical surfaces that keep enlarging until at last they strike 

 the ear. Now there can be no doubt that light likewise reaches us 

 from a luminous substance through some motion caused in the matter 

 lying in the intervening space, — for we have seen above that this can- 

 not take place through transmission of matter from one place to 

 another. 



If, moreover, light requires time for its passage — a matter we shall 

 discuss in a moment — it will then follow that this movement is caused 

 in the substance gradually, and therefore is transmitted, like sound, 

 by surfaces and spherical waves. I call these waves because of their 

 likeness to those formed when one throws a pebble into water, which 

 are examples of gradual propagation in circles, although from a 

 different cause and on a plane surface. 



In regard to the question of light requiring time for its transmis^ 

 sion, let us consider whether there is any experimental evidence 

 against it. 



What experiments we can make here on the earth with sources of 

 light placed at great distances (although indicating that it does not 

 take a sensible time for light to pass over these distances) are subject 

 to the objection that these distances are yet too small, and that we can 

 only argue that the movement of light is enormously fast. M. Des- 

 cartes thought it to be instantaneous and based his opinion upon much 

 better reasons taken from the eclipse of the moon. Yet as I shall 

 make clear, even this evidence is not decisive. I shall state the matter 



