OBJECTIONS TO EMISSION. 237 



each other with great violence, to change each other's 

 directions in a thousand ways, and to mingle together 

 without any order. This difficulty is no doubt specious, 

 but it does not appear insurmountable. 



The chance that two molecules setting out from the 

 same hole should encounter each other, depends both on 

 the absolute diameter of the molecules, and on the inter- 

 vals which separate them. We might then by suitably 

 diminishino- the diameters reduce the chances of encoun- 



O 



ter to nothing. But we have here also in the intervals 

 of the molecules another element, which alone would in 

 a great degree lead to the same conclusion. In fact 

 every sensation of light lasts for a certain time ; the 

 luminous object which has darted its rays into the eye 

 still remains visible (as experiment has proved) at least 

 for an hundredth of a second after the object has dis- 

 appeared. Now, in an hundredth of a second, light has 

 gone through 770 leagues. Thus the luminous mole- 



<D O O 



cules which form each ray may be at 770 leagues inter- 

 val from one another, and nevertheless produce a con- 

 tinuous sensation of light. With such distances what 

 becomes of the repeated clashings spoken of by Euler, 

 and which in any circumstances ought to put a stop to 

 the regular propagation of the rays ? It is almost hu- 

 miliating to see a geometer of so rare a genius believe 

 himself authorized by such futile objections to call the 

 system of emission a mistake of Newton, a gross error, 

 the belief of which, he says, can only be accounted for 

 by recollecting the remark of Cicero, " There is nothing 

 so absurd but that it has been maintained by some phi- 

 losopher." * 



* It has been too common a practice, both with the advocates and 

 the opponents of the wave theory, to rest its defence or its refutation 



