540 DURATION OF VISUAL IMPRESSION. 



Those lines which in the Those lines which in the 



print on the left side are print on the left side are 



farther to the right than in farther to the right than in 



that on the right, appear that on the right, appear 



in the stereoscope raised in the stereoscope raised 



up above the rest. up above the rest. 



Since even the most perfect imitation of a print or drawing 

 differs in some points from the original, the stereoscope is a means 

 of detecting such differences ; for if the original for example, a 

 banknote be compared with the counterfeit, those letters which 

 are not exactly in the same position in both would appear raised or 

 lowered when viewed in the stereoscope. 



The sensation produced by visible objects continues 

 for a very short time after the actual impression from 

 which it results has ceased, as when a body is in rapid 

 motion or is suddenly hidden behind other objects. 

 During the twinkling of the eye, that is the rapid closing 

 of the eyelids for the purpose of diffusing a lubricating 

 fluid over the cornea, we never lose sight of the objects 

 we are viewing. In like manner, when we whirl a 

 glowing splinter of wood with a rapid motion, its glow- 

 ing end will produce a complete circle of light ; if the 

 motion is slower, a luminous arc of a circle will be seen ; 

 but in each case it is obvious that the luminous ex- 

 tremity can only be in one point of the circle at the 

 same instant. 



All such cases depend on the duration of impressions 

 of light on the retina; and the brighter the visible body 

 is, compared with the surrounding objects, the more 



