AN ESSAY ON 



Smith, I took occasion to remark, that the truth 

 of what distinguished it from all others might re- 

 main unshaken, though it were proved, that ob- 

 jects do not appear single, when their pictures 

 occupy any of the corresponding points of the 

 two retinas, since custom might have associated 

 the perceptions of touch, with the sensations of 

 any other parts whatsoever of those membranes. 

 The same observation will not apply with equal 

 justice to the opinion of Dr. Reid. On the 

 contrary, could it be shown, that the places of 

 the two retinas, which represent an object single 

 when each receives its picture, are not the cen- 

 tres, or such others as are similarly situated, an 

 obvious inference would be, that the single ap- 

 pearance of the object is not occasioned by a 

 property in those places, bestowed upon them 

 for this special purpose by nature ; it being 

 reasonable to expect, that such a property 

 should be found, if any where, in those parts of 

 the retinas which are the most like to each 

 other. I have, therefore, reserved till now, the 

 observations which have occurred to me upon 

 this subject, and which, when stated, must at 

 least, raise some doubt concerning what has 

 been regarded as true by Dr. Smith and Dr. 

 Reid, and by almost every other writer on 

 vision since the time of Kepler. 



Anatomists have commonly taught, that the 



