OBJECTIONS TO EMISSION. a 
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 
diminishing the diameters reduce the chances of encoun- 
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- 
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 puta 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 
