84 



EXPERIMENTS, &c. 



be so placed, that the image of the flame must, 

 from the laws of reflection, be regarded as a 

 mere point; though we should now view it 

 with the utmost care, and though there should 

 be in its neighbourhood some well-enlightened 

 object to awaken the attention of the soul, as 

 Mr. Le Cat expresses it, still the seeming sha- 

 dow will move in a direction contrary to the 

 finger. For the image is now so near to the 

 eye, that no exertion we can make is sufficient 

 to bring its rays to a point upon the retina f the 

 picture, therefore, upon that membrane will 

 be formed of rays passing to a focus behind it, 

 which is the only condition necessary for the 

 success of the experiment. Again, if a short- 

 sighted person should place the mirror at the 

 distance of some feet from him, complying in 

 other respects with Mr. Le Cat's instructions, 

 he will constantly observe the shadow to move 

 in the same direction with the finger. For, in 

 his eye, the rays of the image, when at such a 

 distance, must meet before they fall upon the 

 retina. The right side, therefore, of the pic- 

 ture upon that membrane, must be composed, 

 in this case, of rays which enter the eye at the 

 left side of the pupil. Consequently, when these 

 are cut off, the left side of the apparent lu- 

 minous surface must disappear, and the shadow 



