On the Causes of Erect and Single Vision. 411 



by some exterior object, which by the shifting of the axis finds 

 a place on which to paint its rays, " and is equal to the arch 

 between the picture of the given figure on that retina, and the 

 point corresponding to that of the picture on the other retina ;" 

 and which surplus colouring must determine a proportional 

 consciousness to the mind, observing thereby the same rule 

 which determines the " notice of two similar figures, when 

 one eye only is used;... when the apparent distance of two objects 

 seen with one eye is proportional to the arch of the retina which 

 lies between their pictures" and on which an interval of colour- 

 ing is necessarily painted, but which circumstance Dr. Reid did 

 not consider it material to notice *. 



The optical facts to which I have alluded are very shortly and 

 very well expressed in Dr. Reid's " Inquiry f :" The passages in 

 the chapter from which I have partially quoted, and which it 

 may be as well to give entire, are the following ; and I repose 

 on them as stated facts, not containing either hypothesis, 

 opinion, or reasoning. 



" First, — When the axes of both eyes are directed to one 

 point, an object is seen single ; and in this case the two pictures 

 which show the objects single, are in the centres of the retinae. 

 Now in this phenomenon it is evident that the two centres of 

 the retinae are on corresponding points. 



" Secondly, — Pictures of objects seen double, do not fall 

 upon points of the retinae similarly situate with respect to the 

 centres of the retinae. 



" Thirdly, — The apparent distance of two objects seen with 

 one eye, is proportioned to the arch of the retina which lies 

 between their pictures : in like manner the apparent distance 

 of two appearances seen with two eyes, is proportioned to the 

 arch of either retina, which lies between the picture on that 

 retina and the point corresponding to that of the picture on 

 the other retina." 



These facts are valuable for many reasons ; but on no ac- 

 count more so, than because they serve to explain the manner 

 by which nature yields the knowledge of external tangible 

 figure, and the proportional motion which is in relation to it, 

 by means of corresponding varieties of colour. 



Dr. Reid's arguments (although he was in possession of these 

 facts, which might have afforded premises for better reasoning) 

 are altogether inconclusive, not to say puerile; and that on 

 account of his steady adherence to the main object with which 

 he set out upon his " Inquiry," namely, to show upon the prin- 

 ciples of common sense whence comes the knowledge we have 

 of the existence of an external universe : Following up these 



* See Dr. Reid's Inquiry, ch. vi. sec. 13. f Ibid. 



3 G 2 principles, 



