SINGLE VISION. 19 



both these axes to them, it must be evident, to 

 such as are acquainted with the common rules 

 of optics, that the pictures of those objects do 

 not fall upon the centres of the retinas, but 

 more internally ; and, therefore, that the centres 

 and all the other points of those membranes, 

 which by the present system are supposed to 

 represent objects single, do in fact exhibit them 

 double. 



It will be said here, perhaps, that the line* 

 passing from each eye, which we turn to objects 

 when we see them single, is not a production of 

 the common axis of the cornea and globe, but 

 some other, disposed in such a manner, that the 

 pictures of those objects are received by the 

 centres of the retinas. I answer; I readily 

 grant the possibility of the thing, but I assert, 

 at the same time, that we have no proof of it, 



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* I am of opinion, that this line, or at least the line which 

 we turn to objects when we see them most distinctly with 

 one eye, is not the common axis of the globe and cornea. 

 For I find, that, when I plac j the flame of a candle between 

 either of my eyes, and a plane mirror, in such a manner that 

 it may conceal its own image in the mirror from the sight of 

 that eye, or rather that it may be a little below this image, 

 but in the same vertical plane with it, the image of the flame, 

 seen by reflection from the cornea, does not appear upon the 

 middle point of this coat, but upon that point of it which is 

 opposite to the centre of the pupil. 



C 2 



