408 On the Causes of Single and Erect Vision. 



pression of the purple ground must be repeated in two different 

 parts of the sentient retina ; two scarlet lines must be thence 

 impelled, and these made obvious by the intervening horizontal 

 impulse between the circles; for could the intervening space be 

 absorbed, and each point of scarlet coalesce with each point 

 of scarlet, and purple with purple, there would then be but 



four impulsions of colouring, but four varieties, which would 

 be inefficient to the observation of two figures. A coalescence 

 of similar points of colour may perhaps produce a superposi- 

 tion or increment of colour, so as to create a superior bril- 

 liancy in the appearance of the object ; but a coalescence of 

 points cannot give a sense of the separation of points. There- 

 fore for the mind to have a sense of one figure, there must be two 

 consciousnesses of colour ; andto have a sense of two figures, 

 there must he five distinguishable consciousnesses of colour. 

 However often one colour only be repeated, there will be no 

 figure ; however often two similar colours be repeated, (no in- 

 tervening one being supposed,) hut one figure will arise; whilst 

 to entertain a sense of two figures, jfe impulsions are necessary. 

 I hold it therefore as an axiom in the laws of vision, that the 

 repetition of similar impulsions of colour will not yield a num- 

 ber of figures equal to the number of such impulsions ; but that 

 the number of figures perceived, arises from those proportional 

 intervals of the hnpidsions of colour, which must vary in a certain 

 ratio to the number of figures impelled. 



However, therefore, the number of eyes may be multiplied, 

 the mind can have no consciousness of any additional number 

 of figures, whilst only similar impulsions of colour are yielded 

 to it. 



Let us enter into some further detail. If one scarlet circle 

 on the purple ground be painted on the corresponding points 

 of two retinae, and thence impelled to the mind, there can still 

 only arise the sense of one scarlet circle ; for there have existed 

 but four physical varieties on the retinae, and but four varieties 

 have been impelled to the mind : and it has been proved that 



five physical varieties are necessary to exist upon a retina, or 

 upon retinae however numerous, in order to impel correspond- 

 ing consciousnesses to the mind, and which would be necessary 

 for the mental apprehension of two figures. Let the figures 

 without an intervening horizontal colour be supposed to be 



2 4 

 marked thus, d)-JL(|). The intervening horizontal colouring 



necessary to separate the painting of two figures on the retinae, 

 is not painted on either retina ; it does not exist ; and therefore 

 no conscious separation of two figures can possibly arise to 



the 



