Prof. Wheatstone on the Physiology of Vision. 251 



attracted sufficient attention to have been made the subject of 

 philosophic observation. It was one of the earliest facts which 

 drew my attention to the subject I am now treating. 



Dr. Smith * was very much puzzled by an effect of binocular 

 perspective which he observed, but was unable to explain. He 

 opened a pair of compasses, and while he held the joint in his 

 hand, and the points outwards and equidistant from his eyes, 

 and somewhat higher than the joint, he looked at a more distant 

 point; the compasses appeared double. He then compressed 

 the legs until the two inner points coincided ; having done this 

 the two inner legs also entirely coincided, and bisected the angle 

 formed by the outward ones, appearing longer and thicker than 

 they did, and reaching from the hand to the remotest object in 

 view. The explanation offered by Dr. Smith accounts only for 

 the coincidence of the points of the compasses, not for that of 

 the entire leg. The effect in question is best seen by employing 

 a pair of straight wires, about a foot in length. A similar obser- 

 vation, made with two flat rulers, and afterwards with silk threads, 

 induced Dr. Wells to propose a new theory of visible direction in 

 order to explain it, so inexplicable did it seem to him by any of 

 the received theories. 



§9. 



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 employed, and that the most 

 vivid belief of the solidity of an object of three dimensions arises 

 from two different perspective projections of it being simulta- 

 neously 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 

 tanner, become n:ss liable to lead the judgement 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, heroines less and less as the object is seen 



at a greater distance before the eyeB, and entirely ceases when it 

 distant thai the optic axes are parallel while regarding it. 

 E with both eyes all objects beyond this distance precisely 

 * System of Optics, vol. ii. p. .'iSH. nnd r. §96. 



