MR. WHEATSTONE ON THE PHYSIOLOGY OP VISION. 375 



The stereoscope is represented by figs. 8. and 9 ; the former being a front view, 

 and the latter a plan of the instrument. 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 up- 

 right 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 hereafter 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 faci- 

 litate 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 

 corresponding horizontal lines shall be on the same level : these pannels are capable 

 of sliding backwards and forwards in grooves on the upright boards D D'. The ap- 

 paratus 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 right 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 

 intersection 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 different 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 

 magnitude 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 al- 

 teration in the apparent magnitude of the binocular images, when these usual relations 

 are disturbed, will be discussed in another paper of this series, with a variety of re- 

 markable phenomena depending thereon. In all the experiments detailed in the pre- 

 sent memoir I shall suppose these relations to remain undisturbed, and the optic axes 

 to converge about six or eight inches before the eyes. 



If the pictures are all drawn to be seen with the same inclination of the optic axes, 

 the apparatus may be simplified by omitting the screw r / and fixing the upright 

 boards D D' at the proper distances. The sliding pannels may also be dispensed with, 

 and the drawings themselves be made to slide in the grooves. 



