474 Dr Alison on Single and Correct Vision, by means of 



ject touched by us, had produced, not two merely, but two thou- 

 sand separate images in our eyes, erect or inverted, or in any in- 

 termediate degree of inclination, the visual feeling thus excited 

 would still have accompanied the touch of a single object ; and if 

 only it had accompanied it uniformly, the single object would 

 have been suggested by it, precisely in the same manner as it is 

 now suggested by the particular visual feeling that attends the 

 double inverted image."* 



But, with all deference to this illustrious metaphysician, I will 

 take the liberty of stating, that this view of the subject had been 

 previously fully considered, and, as far as I can judge, completely 

 set aside by Dr Re id, at least in reference to single vision by 

 two images on the retina? ; and that, not by any abstract reason- 

 ing, but by appeal to facts. 



If it were only by experience, and association with the per- 

 ceptions of touch, that we learned that any object placed before 

 the eyes, and seen by two images, is nevertheless single, it seems 

 prima facie reasonable to conclude, that we should never see an 

 object double, which we know by touch to be single ; whereas 

 we all know, that if, by pressure on the ball of one eye, or by 

 any other means, we direct the axes of the two eyes to different 

 points in an object, we immediately see it double, and cannot, by 

 any means, avoid seeing it double, so long as that condition of 

 the eyes continues, notwithstanding the full conviction, derived 

 from touch, of its being single. 



The only answer that 1 can conceive to this is, that the asso- 

 ciation, by which we are informed of an object of sight being 

 single, is formed with the natural and healthy state of the vision 

 of that object, when the axes of the two eyes are directed to the 

 same point in the object, and its images are formed on corre- 

 sponding points of the retinae of the two eyes ; and that when 

 its images are formed on dissimilar points of the retinae, the diffe- 



* Lect. 29. 



