380 MR. WHEATSTONE ON THE PHYSIOLOGY OP VISION. 



The preceding experiments render it evident that there is an essential difference in 

 the appearance of objects when seen with two eyes, and when only one eye is em- 

 ployed, and that the most vivid belief of the solidity of an object of three dimensions 

 arises from two different perspective projections of it being simultaneously presented 

 to the mind. How happens it then, it may be asked, that persons who see with only 

 one eye form correct notions of solid objects, and never mistake them for pictures ? 

 and how happens it also, that a person having the perfect use of both eyes, perceives 

 no difference in objects around him when he shuts one of them ? To explain these 

 apparent difficulties, it must be kept in mind, that although the simultaneous vision 

 of two dissimilar pictures suggests the relief of objects in the most vivid manner, yet 

 there are other signs which suggest the same ideas to the mind, which, though more 

 ambiguous than the former, become less liable to lead the judgment astray in pro- 

 portion to the extent of our previous experience. The vividness of relief arising from 

 the projection of two dissimilar pictures, one on each retina, becomes less and less as 

 the object is seen at a greater distance before the eyes, and entirely ceases when it is 

 so distant that the optic axes are parallel while regarding it. We see with both eyes 

 all objects beyond this distance precisely 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 person 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 unconsciously 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 consi- 

 derations. The mind associates with the idea of a solid object every different projec- 

 tion of it which experience 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 com- 

 pletely characterized. While the object remains fixed, at every movement of the 

 head it is viewed from a different point of sight, and the picture on the retina conse- 

 quently 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, especially 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 saine 

 lines^ shades and colours on the retina, as the more distant scene which it represents 



