SINGLE VISION. 25 



then should these objects appear, if, instead of 

 being viewed alternately, each by the eye in 

 the axis of which it is placed, they were seen 

 by the two together ; the positions and internal 

 states of the eyes being in both cases the same ? 

 Dr. Reid must answer ; They will possess but 

 one visible place, since their pictures fall upon 

 the centres of the two retinas, points endowed 

 with the original property of representing ob- 

 jects single. But where is this one place to be 

 found ? In the axis of the right eye, or in that 

 of the left, or between the two ? In any of these 

 cases, or in any other that can be imagined, the 

 law of visible direction, so much insisted upon 

 by Dr. Reid, that objects appear in the per- 

 pendiculars to their pictures upon the retina, 

 and in truth every other law of visible direction 

 hitherto published, must be suspended with re- 

 spect to one or both eyes ; unless, indeed, the 

 united object be referred to the intersection of 

 the optic axes, about an inch or two from the 

 face. This, I believe, Dr. Reid would not 

 readily admit ; but if he should, another case 

 of squinting may be imagined, in which the 

 optic axes recede from each other, and where 

 the same reasoning will apply without the pos- 

 sibility of its force being thus eluded. It now 

 remains for me to mention, that the experiment 

 here stated by the way of supposition, in which 



