On the Causes of Single and Erect Vision. 407 



cation only is presented, as in one eye supposed. Should it be 

 asked, whence is it that such a proportional variety is not 

 presented to the mind ? The answer which the premises and 

 optical experiments equally support, is, because the impulse 

 upon each eye {when the axes of both are directed to the same 

 point or object,) being precisely alike, there is no variety of 

 colouring painted upon either eye, equal to the production of 

 that variety of perception, necessary to yield the ideas of two 

 objects separated from each other, between their interior and 

 horizontal edges. 



Let the letter A, for instance, be painted upon one eye, and 

 the perception of its figure arises in the mind, from the points 

 of distinction between the black letter and the white around 

 it : there is a sense of difference created. Place it on similar 

 points of two retinae, and each point of the figure painted on 

 each retina will yield to the mind but one point of conscious 

 black against one point of conscious white ; and not two points 

 of black against two points of white, because there is no inter- 

 vening white painted on either retina, which can yield a con- 

 sciousness of the separation of the two A's to a distance from 

 each other, thus, A- A. 



The white space between the two A's is not painted on 

 either retina. How then can any idea of it arise in the mind ? 



If, in order to render these ideas more intelligible, we ana- 

 lyse with still greater nicety the question, why we see dupli- 

 cates of similar figures with one eye only supposed, it will at 

 once appear obvious why we can perceive but duplicates of 

 such figures, instead of quadruplicates,v/hen two eyes are used. 



Now if one eye should see but one colour only, it is sup- 

 posed to be granted that there could be no sense of any de- 

 fined figure whatever : one impulse therefore yields not figure. 



If one and the same colour should be seen by two eyes, it 

 must still be acknowledged there would be no figure : two 

 similar impulses therefore cannot give the sense of figure to 

 the mind. Now upon one colour (say a purple ground) painted 

 upon one retina, mark a scarlet circle O ; a sense of one figure 

 will immediately arise from two varieties of colour being car- 

 ried to the mind, viz. a line of demarcation to the purple 

 ground by the scarlet circle : two impulsions, or two varieties 

 of colouring, are therefore necessary to the perception of one 

 figure. Again, if with one eye given, I wish to see two scarlet 

 circles upon the purple ground, what must I do? Will four 

 impulsions yield two similar figures ? I answer, No. There 

 must he five impulsions in order to convey to the mind the 



2 5 4 

 sense of two figures : there must be ®~ip®; that is, the im- 

 pression 



