IN OPTICS. 81 



principles of vision, I think it worth while to 

 show, in a few words, that it is a direct conse- 

 quence of the very doctrine Mr. Le Cat means 

 to overthrow by its means. 



It would be proper, indeed, to mention before- 

 hand, the opinion of the Abbot Derochon ; but 

 this I must, notwithstanding, omit doing, as it 

 could not be understood without the figure by 

 which he has illustrated it. I shall observe, 

 however, respecting it, first, that it requires the 

 side of the ringer next to the eye, to be without 

 the least illumination ; whereas the experiment 

 will succeed, whether it be illuminated or not : 

 secondly, that, according to it, the experiment 

 ought to succeed equally well, whether the 

 image of the flame in the mirror be seen as a 

 point, or as a surface; though, in truth, it 

 never does succeed, except in the latter cases : 

 thirdly, that the apparent shadow of the finger 

 is always much larger than it ought to be, were 

 it seen by reflection, as the Abbot thinks : 

 fourthly, that, while the eye, mirror, flame, and 

 finger, remain in the same positions, the shadow 

 seems at one time larger than at another, owing 

 to the different degrees of relaxation in the eye ; 

 but that this, for the reason just mentioned, 

 ought never to happen, according to his theory : 

 fifthly, that agreeably to his own reasoning, the 



