234 FRESNEL. 
ence the most illustrious names could be authorities capa- 
ble of determining the point. 
If, however, it astonish us to see men of such great 
genius thus divided, I would say that in their times the 
question in dispute cowld not be resolved ; that the neces- 
sary experiments were wanting ; and that then the two 
different theories of light were not logical deductions 
from facts, but, if I may so express myself, simple mat- 
ters of persuasion; and that, in a word, the gift of infal- 
libility is not granted even to the most skilful, if they 
transgress the bounds of observation, and, abandoning 
themselves to conjecture, desert the strict and sure path 
by which science advances in our age on reasonable prin- 
ciples, and by which it has been enabled to make such 
incontestable progress. Before we review the great in- 
roads which have been recently made on the theory of 
emission, it will be perhaps convenient to cast a glance 
over the vigorous attacks of which it was the object, in 
the writings of Euler, of Franklin, and others; and to 
show that the partisans of Newton might then, without 
looking forward too much, have considered the solution 
as adjourned for a long period. ‘The effects which a 
cannon ball can produce depend so directly on its mass 
and its velocity jointly, that we can, without altering 
them, change at pleasure one of these elements, provided 
we make the others change in an inverse ratio. Thus a 
ball of two kilogrammes may overthrow a wall; a ball of 
one kilogramme will also overthrow it, provided we im- 
press on it a velocity double of the former. If the weight 
of the ball were reduced to 51,th or A 9th of its original 
amount, to produce the same effect we must give a veloc- 
ity ten times or one hundred times as great. Now we 
know that the velocity of a cannon ball-is the 640,000th 
