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 could 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 -^th or T ^th 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 



