Prof. Wheatstone on the Physiology of Vision. 245 



lines be drawn on two pieces of card, at the respective lateral di- 

 stances at which they appear to each eye, and these cards be after- 

 wards viewed by either of the means above directed, the observer 

 will no longer see lines on a plane surface, as each card sepa- 

 rately shows ; but two lines will appear, one nearer to him than 

 the other, precisely as the original vertical lines themselves. 

 Again, if a straight wire be held before the eyes in such a posi- 

 tion that one of its ends shall be nearer to the observer than the 

 other is, each eye separately referring it to a plane perpendicular 

 to the common axis, will see a line differently inclined; and then 

 if lines having the same apparent inclinations be drawn on two 

 pieces of card, and be presented to the eyes as before directed, 

 the real position of the original line will be correctly perceived 

 by the mind. 



In the same manner the most complex figures of three dimen- 

 sions may be accurately represented to the mind, by presenting 

 their two perspective projections to the two retinae. But I shall 

 defer these more perfect experiments until I describe an instru- 

 ment which will enable any person to observe all the phsenomena 

 in question with the greatest ease and certainty. 



In the instruments above described, the optic axes converge 

 to some point in a plane before or beyond that in which the 

 objects to be seen are situated. The adaptation of the eye, 

 which enables us to see distinctly at different distances, and 

 which habitually accompanies every different degree of conver- 

 gence of the optic axes, does not immediately adjust itself to the 

 new and unusual condition ; and to persons not accustomed to 

 experiments of this kind, the pictures will either not readily 

 unite, or will appear dim and confused. Besides this, no object 

 can be viewed according to either mode when the drawings ex- 

 ceed in breadth the distance of the two points of the optic axes 

 in which their centres are placed. 



These inconveniences are removed by the instrument I am 

 about to describe; the two pictures (or rather their reflected 

 images) are placed in it at the true concourse of the optic axes, 

 the local adaptation of the eye preserves its usual adjustment, 

 the appearance of lateral images is entirely avoided, and a large 

 field of view for each eye is obtained. The frequent reference I 

 shall have occasion to make to this instrument will render it 

 convenient to give it a specific name; I therefore propose that 

 it be called a stereoscope, to indicate its property of representing 

 solid figures. 



§3. 



The stereoscope is represented by figs. 8 and 9; the former 

 being a front view, and the latter a plan of the instrument. 



