EXPERIMENTS, &c. 



ARTICLE II. 



On a supposed Consequence of the Duration of Impres- 

 sions upon the Retina; and the Effects of accurate 

 Vision being confined to a single Point ofiliat Mem- 

 brane, 



FEW things, at first, appear more incredible 

 to a person, not conversant in optics, than that 

 he does not, at any one time, see distinctly a 

 surface larger than the head of a pin. After 

 he is convinced, by proper trials, of the truth 

 of this, he naturally asks, Whence comes it 

 then, that, in ordinary vision, I seem to view 

 distinctly so many objects at once ? I go into 

 a crowded street, and I fancy I have an accu- 

 rate perception by sight, of men, houses, car- 

 riages, and many other things, all at the same 

 time 5 whence proceeds this illusion ? 



Only one answer, as far as I know, has been 

 given to this question. The impressions made 

 upon the retina by external objects, do not, it 

 is said, immediately cease, along with the re- 

 ception of the rays which flow from them ; and, 

 as in the ordinary mode of vision, the eye is 

 continually passing from object to object, the 

 impression left by a former one may be still 

 vivid, though the eye be directed to another j 



