252 Prof. Wheatstone on the Physiology of Vision. 



as we see near objects with a single eye ; for the pictures on the 

 two retinae are then exactly similar, and the mind appreciates no 

 difference whether two identical pictures fall on corresponding 

 parts of the two retinae, or whether one eye is impressed with only 

 one of these pictures. A person deprived of the sight of one 

 eye sees therefore all external objects near and remote, as a per- 

 son with both eyes sees remote objects only, but that vivid effect 

 arising from the binocular vision of near objects is not perceived 

 by the former ; to supply this deficiency he has recourse uncon- 

 sciously to other means of acquiring more accurate information. 

 The motion of the head is the principal means he employs. That 

 the required knowledge may be thus obtained will be evident 

 from the following considerations. The mind associates with the 

 idea of a solid object every different projection of it which expe- 

 rience has hitherto afforded ; a single projection may be ambi- 

 guous, from its being also one of the projections of a picture, or 

 of a different solid object ; but when different projections of the 

 same object are successively presented, they cannot all belong to 

 another object, and the form to which they belong is completely 

 characterized. While the object remains fixed, at every move- 

 ment of the head it is viewed from a different point of sight, and 

 the picture on the retina consequently continually changes. 



Every one must be aware how greatly the perspective effect of 

 a picture is enhanced by looking at it with only one eye, espe- 

 cially when a tube is employed to exclude the vision of adjacent 

 objects, whose presence might disturb the illusion. Seen under 

 such circumstances from the proper point of sight, the picture 

 projects the same lines, shades and colours on the retina, as the 

 more distant scene which it represents would do were it substi- 

 tuted for it. The appearance which would make us certain that 

 it is a picture is excluded from the sight, and the imagination 

 has room to be active. Several of the older writers erroneously 

 attributed this apparent superiority of monocular vision to the 

 concentration of the visual power in a single eye*. 



There is a well-known and very striking illusion of perspective 

 which deserves a passing remark, because the reason of the effect 

 does not appear to be generally understood. When a perspective 

 of a building is projected on a horizontal plane, so that the point 

 of sight is in a line greatly inclined towards the plane, the 

 building appears to a single eye placed at the point of sight, to 

 be in bold relief, and the illusion is almost as perfect as in the 

 binocular experiments described in §§ 2,3,4. This effect wholly 



* "We see more exquisitely with one eye shut than with both, because 

 the vital spirits thus unite themselves the more, and become the stronger : 

 for we may find by looking in a glass whilst we shut one eye, that the pupil 

 of the other dilates." — Lord Bacon's Works, Sylvu Sylvarvm, art. Vision. 



