246 Prof. "Wheatstone on the Physiology of Vision. 



A A' are two plane mirrors, about four inches square, inserted in 

 frames, and so adjusted that their backs form an angle of 90° 

 with each other ; these mirrors are fixed by their common edge 

 against an upright B, or which was less easy to represent in the 

 drawing, against the middle line of a vertical board, cut away in 

 such manner as to allow the eyes to be placed before the two 

 mirrors. C C are two sliding boards, to which are attached the 

 upright boards D D', which may thus be removed to different 

 distances from the mirrors. In most of the experiments here- 

 after to be detailed, it is necessary that each upright board shall 

 be at the same distance from the mirror which is opposite to it. 

 To facilitate this double adjustment, I employ a right and a left- 

 handed wooden screw, r I; the two ends of this compound screw 

 pass through the nuts e e', which are fixed to the lower parts of 

 the upright boards D D', so that by turning the screw pin p one 

 way the two boards will approach, and by turning it the other 

 they will recede from each other, one always preserving the same 

 distance as the other from the middle line /. E E' are pannels, 

 to which the pictures are fixed in such manner that their corre- 

 sponding horizontal lines shall be on the same level : these pan- 

 nels are capable of sliding backwards and forwards in grooves on 

 the upright boards D D'. The apparatus having been described, 

 it now remains to explain the manner of using it. The observer 

 must place his eyes as near as possible to the mirrors, the light 

 eye before the right-hand mirror, and the left eye before the left- 

 hand mirror, and he must move the sliding pannels E E' to or 

 from him until the two reflected images coincide at the intersec- 

 tion of the optic axes, and form an image of the same apparent 

 magnitude as each of the component pictures. The pictures will 

 indeed coincide when the sliding pannels are in a variety of dif- 

 ferent positions, and consequently when viewed under different 

 inclinations of the optic axes ; but there is only one position in 

 which the binocular image will be immediately seen single, of 

 its proper magnitude, and without fatigue to the eyes, because 

 in this position only the ordinary relations between the magni- 

 tude of the pictures on the retina, the inclination of the optic 

 axes, and the adaptation of the eye to distinct vision at different 

 distances are preserved. The alteration in the apparent magni- 

 tude of the binocular images, when these usual relations are 

 disturbed, will be discussed in another paper of this series, with 

 a variety of remarkable phrenomena depending thereon. In all 

 the experiments detailed in the present memoir I shall suppose 

 these relations to remain undisturbed, and the optic axes to con- 

 verge about six or eight inches before the eyes. 



If the pictures are all drawn to be seen with the same inclina- 

 tion of the optic axes, the apparatus may be simplified by omit- 



