PEKCEPTION OF A MOVING OBJECT. 



retina, after the light from the object ceases to act, is also mani- 

 fested by the fact, that the continual winking of the eyes for the 

 purpose of lubricating the eye-ball by the eye-lid does not 

 intercept our vision. If we look at any external objects, they 

 never cease for a moment to be visible to us, notwithstanding 

 the frequent intermissions which take place in the action of light 

 upon the retina in consequence of its being thus intercepted by 

 the eye-lid. 



53. If a lighted stick be turned round in a circle in a dark 

 room, the appearance to the eye will be a continuous circle of 

 light ; for in this case the impression produced upon the retina by 

 the light, when the stick is at any point of the circle, is retained 

 until the stick returns to that point. 



54. In the same manner, a flash of lightning appears to the 

 eye as a continuous line of light, because the light emitted at any 

 point of the line remains upon the retina until the cause of the 

 light passes over the succeeding points. In the same manner, 

 any objects moving before u the eye with such a velocity that the 

 retina shall retain the impression produced at one point in the line 

 of its motion until it passes through the other points, will appear 

 as a continuous line of light or colour. 



55. But to produce this effect, it is not enough that the body 

 change its position so rapidly that the impression produced at one 

 point of its path continues until its arrival at another point ; it is 

 necessary, also, that its motion should not be so rapid as to make 

 it pass from any of the positions which it successively assumes 

 before it has time to impress the eye with a perception of it ; for 

 it must be remembered, as has been already explained, that the 

 perception of a visible object presented to the eye, though rapid, 

 is not instantaneous. 



The object must remain present before the organ of vision a 

 certain definite time, and its position must continue upon the 

 retina during such time, before any perception of it is obtained. 

 Now, if the body move from its position before the lapse of this 

 time, it necessarily follows that no perception of its presence, 

 therefore, will be obtained. If, then, we suppose a body moving 

 so rapidly before the eye that it remains in no position long 

 enough to produce a perception of it, such object will not be seen. 



56. Hence it is that the ball discharged from a cannon passing 

 transversely to the line of vision is not seen ; but if the eye be 

 placed in the direction in which the ball moves, so that its angular 

 motion round the eye as a centre will be slow notwithstanding its 

 great velocity, it will be visible, because, however rapid its real 

 motion through space, its angular motion with respect to the eye 

 (and consequently of the image of its picture on the retina) will 



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