On the Causes of Erect and Single Vision. 413 



terior horizontal edges of the two A's, which is necessary to 

 yield the ideas of their separate figures. 



But to return " to the explanation of the Cause of Erect and 

 Single Vision, published by the Society for the Diffusion of 

 Useful Knowledge* ;" — it appears to me to be as much at va- 

 riance with abound, metaphysical, demonstrable conclusion 

 concerning the nature of the perception of visible figure by the 

 mind, as are the authors to whom I have alluded ; and as much 

 so with an acknowledged law, — with a proved physical fact, in 

 respect to the time required for the motion of light. 



The author of the " Explanation of the Cause of Erect and 

 Single Vision," saysf: "As the lines of visible direction cross 

 each other at the centre of visible direction, an erect object is the 

 necessary result of an inverted image ;" but this is not the same 

 thing with the perception of an erect object. If it be said the 

 word perception is understood though not expressed, then the 

 mind is supposed to see the very erect object, out of itself, at a 

 distance from itself; that is, the mind feels colour, perceives 

 visible figure (its result), there, inhere it is not, which is im- 

 possible. 



Again, it is a known fact, that the light emitted from the sun, 

 employs about eight minutes in its journey to the earth. Now 

 let an object be seen at that distance in an erect position, but 

 the moment after its light is effused, let it be obliterated: the 

 mind will still see it erect, eight minutes after its annihilation, 

 how then shall it signify the drawing of any rays back through 

 a centre, towards the place of an obliterated object, which once 

 stood there erect ? The author's explanation is little more than 

 the very circumstance in question, re-stated by an inversion 

 of words. M An erect object (at a distance) is the result of an 

 inverted image on the retina by the crossing of rays at a cen- 

 tre ;" is merely saying over again, that an inverted image on 

 the retina is the result of an erect object at a distance, when rays 

 cross at a centre. 



The question still remains untouched and unexplained; 

 namely, why does the mind perceive an erect image, the re- 

 sult of an inverted image, which inverted image is the proxi- 

 mate cause of vision, and not the erect object which might be 

 obliterated without affecting the mental consciousness of it? 

 The only answer appears to me to be that, which I have for- 

 merly stated in my " Essay on Single and Erect Vision ;" viz. 

 " Inversion of figure is merely a relative quality: when all rays 

 from every object within the sphere of vision become inverted on 

 the retina, there truly can be no mental consciousness of any in- 



* Optics, part ii. " Library of Useful Knowledge." f Ibid. 



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