372 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



be seen single only when their images fall on corresponding points of the two retinae, 

 an hypothesis which will be hereafter discussed, if the consideration ever arose in 

 their minds, it was hastily discarded under the conviction, that if the pictures pre- 

 sented to the two eyes are under certain circumstances dissimilar, their differences 

 must be so small that they need not be taken into account. 



It will now be obvious why it is impossible for the artist to give a faithful repre- 

 sentation of any near solid object, that is, to produce a painting which shall not be 

 distinguished in the mind from the object itself. When the painting and the object 

 are seen with both eyes, in the case of the painting two similar pictures are projected 

 on the retinae, in the case of the solid object the pictures are dissimilar ; there is 

 therefore an essential difference between the impressions on the organs of sensation 

 in the two cases, and consequently between the perceptions formed in the mind ; the 

 painting therefore cannot be confounded with the solid object. 



After looking over the works of many authors who might be expected to have made 

 some remarks relating to this subject, I have been able to find but one, which is in 

 the Trattato della Pittura of Leonardo da Vinci *. This great artist and ingenious 

 philosopher observes, " that a painting, though conducted with the greatest art and 

 finished to the last perfection, both with regard to its contours, its lights, its shadows 

 and its colours, can never show a relievo equal to that of the natural objects, unless 

 these be viewed at a distance and with a single eye. For," says he, " if an object C 

 (Plate X. fig. 1.) be viewed by a single eye at A, all objects in the space behind it, in- 

 cluded as it were in a shadow E C F cast by a candle at A, are invisible to the eye 

 at A ; but when the other eye at B is opened, part of these objects become visible to 

 it; 'those only being hid from both eyes that are included, as it were, in the double 

 shadow C D, cast by two lights at A and B, and terminated in D, the angular space 

 E D G beyond D being always visible to both eyes. And the hidden space C D is so 

 much the shorter, as the object C is smaller and nearer to the eyes. Thus the object C 

 seen with both eyes becomes, as it were, transparent, according to the usual definition 

 of a transparent thing ; namely, that which hides nothing beyond it. But this cannot 

 happen when an object, whose breadth is bigger than that of the pupil, is viewed by 

 a single eye. The truth of this observation is therefore evident, because a painted 

 figure intercepts all the space behind its apparent place, so as to preclude the eyes 

 from the sight of every part of the imaginary ground behind it." 



Had Leonardo da Vinci taken, instead of a sphere, a less simple figure for the 

 purpose of his illustration, a cube for instance, he would not only have observed that 

 the object obscured from each eye a different part of the more distant field of view, 

 but the fact would also perhaps have forced itself upon his attention, that the object 

 itself presented a different appearance to each eye. He failed to do this, and no sub- 

 sequent writer within my knowledge has supplied the omission ; the projection of two 



* See also a Treatise of Painting, p. 178. London, 1721 ; and Dr. Smith's Complete System of Optics, 

 Tol. ii. r. 244, where the passage is quoted. 



