MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 371^ 



form will appear; in both these experiments we have the singular phenomenon of the 

 conversion of a single plane outline into a figure of three dimensions. To render the 

 binocular object more distinct, concave lenses may be applied to the eyes ; and to 

 prevent the two lateral images from being seen, screens may be placed at D and D'. 



An effect of binocular perspective may be remarked in a plate of metal, the surface 

 of which has been made smooth by turning it in a lathe. When a single candle is 

 brought near such a plate, a line of light appears standing out from it, one half being 

 above, and the other half below the surface ; the position and inclination of this line 

 changes with the situation of the light and of the observer, but it always passes 

 through the centre of the plate. On closing the left eye the relief disappears, and the 

 luminous line coincides with one of the diameters of the plate ; on closing the right 

 eye the line appears equally in the plane of the surface, but coincides with another 

 diameter ; on opening both eyes it instantly starts into relief*. The case here is ex- 

 actly analogous to the vision of two inclined lines (fig. 10.) when each is presented 

 to a different eye in the stereoscope. It is curious, that an effect like this, which 

 must have been seen thousands of times, should never have attracted sufficient at- 

 tention 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 -j- 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 com- 

 passes appeared double. He then compressed the legs until the two inner points co- 

 incided ; 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 observation, 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. 



* The luminous line seen by a single eye arises from the reflection of the light from each of the concentric 

 circles produced in the operation of turning ; when the plate is not large the arrangement of these successive 

 reflections does not differ from a straight line. 



t System of Optics, vol. ii. p. 388. and r. 526. 



3 c 2 



