IN OPTICS. 79 



To have the same thing proved in another way, 

 I desired a person to turn quickly round, till he 

 became very giddy ; then to stop himself and 

 look stedfastly at me. He did so, and I could 

 plainly see, that, although he thought his eyes 

 were fixed, they were in reality moving in their 

 sockets, first toward one side, and then toward 

 the other. 



The last instance of visible motion I shall 

 notice, is one which has been mentioned by 

 Mr. Le Cat, in the following words * : " Place 

 a lighted candle at a moderate distance from a 

 polished body of considerable convexity, so that 

 the image of the flame, which is seen by reflec- 

 tion from it, may appear as a small luminous 

 point. The experiment will succeed better, if 

 the direct rays of the flame be intercepted from? 

 the sight. Close, after this, one eye, and view 

 the luminous point in a careless way, (en revanf) 

 that is to say, with the eye in a relaxed or dilated 

 state. The point will then be seen enlarged and 

 radiated. If you bring now your finger to the 

 right of the eye which i& open, and gradually 

 move it toward the left, in order to conceal the 

 luminous point from this eye, you will distinctly 

 perceive the shadow of your finger to proceed 

 from left to right, and to pass over the point in 



* Trait6 des Sens. p. 419- 



