242 Prof. Wheatstone on the Physiology of Vision. 



each eye successively while the other is closed. Plate VIII. fig. 13 

 represents the two perspective projections of a cube; b is that 

 seen by the right eye, and a that presented to the left eye j the 

 figure being supposed to be placed about seven inches imme- 

 diately before the spectator. 



The appearances, which are by this simple experiment ren- 

 dered so obvious, may be easily inferred from the established 

 laws of perspective ; for the same object in relief is, when viewed 

 by a different eye, seen from two points of sight at a distance 

 from each other equal to the line joining the two eyes. Yet 

 they seem to have escaped the attention of every philosopher and 

 artist who has treated of the subjects of vision and perspective. 

 I can ascribe this inattention to a phenomenon leading to the 

 important and curious consequences, which will form the subject 

 of the present communication, only to this circumstance ; that 

 the results being contrary to a principle which was very gene- 

 rally maintained by optical writers, viz. that objects can be seen 

 single only when their images fall on corresponding points of 

 the two retina?, an hypothesis which will be hereafter discussed, 

 if the consideration ever arose in their minds, it was hastily dis- 

 carded under the conviction, that if the pictures presented to the 

 two eyes are under certain circumstances dissimilar, their differ- 

 ences 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 representation 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 pic- 

 tures are projected on the retinae, in the case of the solid object 

 the pictures are dissimilar ; there is therefore an essential differ- 

 ence 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 VII. fig. 1] be viewed by a single eye at 



* See also a Treatise of Painting, p. 178. London,l/21 ; and Dr. Smith's 

 Complete System of Optics, vol. ii. r. 244, where the passage is quoted. 



