4-12 On the Causes of Erect and Single Vision. 



principles, he placed visible figure beyond the body, at a di- 

 stance from the perceiving mind, denying it to consist either 

 in a sensation, impression, or idea, — and as possible to be seen 

 without the intervention of colour *. 



It appears to me strange, when contradiction is stamped 

 upon the very expressions which convey these ideas, that Dr. 

 Reid's notions should seem to be the data for the reasonings 

 of the author of the " Explanation of -Erect and Single Vision" 

 published in the " Library of Useful Knowledge." 



I must however, in common honesty, here take notice of an 

 objection which I have known to be made to the views I en- 

 tertain on this subject : it is, " that we see objects in different 

 directions by either eye, when the other is alternately opened 

 or closed." This objection appears to me perfectly nugatory, 

 when it is considered, that both eyes being opened together, 

 they are allowed by the condition of the question to be directed 

 to one point; in which case neither of them can be directed to 

 any point beyond that point; it would be a contradiction in 

 terms to admit it. The axes cannot cross each other, and look 

 at points beyond the given point, and that with a separate con- 

 sciousness in the mind of so doing ; for then these would not 

 be merely one given point, but three given points; and the 

 figure, the cause of whose single vision is in question, would 

 be supposed to be placed, and at the same 

 time supposed not to be placed, at the junc- 

 tion of the axes. For instance, when two 

 eyes are directed to A, the left eye cannot 

 be turned to B, look by itself at 13, and the 

 right eye at the same time be made to look 

 by itself at C. Experience shows this to 

 be an impossibility ; but when either eye is 

 shut, the other may be moved in any direction we please. 

 However, were I in error in this statement, the argument of 

 my objector would by no means be conclusive against my 

 doctrine of single vision, provided only that A be placed at 

 the junction of the axes ; for the utmost which could happen 

 would be, that A plus B, plus C would appear to the mind ; but 

 not two A's (two B's and two C's), because there would still 

 be only a superposition, or increment of the colouring of A. 



The central point of the colour of A would coincide on 

 each retina; the whole of the rest of the colouring in relation 

 to it would be painted on corresponding points, and coincide 

 on their respective retinae ; and there could in no wise arise 

 that proportional variety of colour, painted between the in- 



* See Dr. Reid's " Inquiry," ch. vi. sect. 12. p. 135. 12mo. 



terior 



