IN OPTICS. 83 



side of the pupil, and its left side is formed of 

 the rays entering at the left side of the pupil j 

 and the like must be true of its upper and lower 

 parts. Should we then begin to move a finger 

 from right to left across the eye, the rays form- 

 ing the right side of the picture must be first 

 intercepted. But from the known fact, that 

 the points of an external object are always in 

 an inverted position, with respect to the parts 

 of the retina, by the affections of which they 

 are suggested, when the right side of the picture 

 there is effaced, the left side of the external 

 object it suggests must disappear. And for 

 the same reason, if the motion of the finger be 

 continued from right to left across the eye, the 

 other parts of the luminous surface in the mirror 

 will successively vanish from left to right, and 

 thereby furnish the appearance of a shadow 

 passing over it in that direction. In like man- 

 ner, it maybe shown, that if the finger proceeds 

 from left to right, from above downward, or 

 from below upward, the shadow must move the 

 opposite way. 



That this is the true explanation of Mr. Le 

 Cat's experiment, is, I think, plain, both from 

 its intrinsic evidence, and the following consi- 

 derations : If the mirror be brought within 

 four or five inches of the eye, and the candle 



