374 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



viewed by that eye which is directed towards it, the observer will perceive a figure of 

 three dimensions, the exact counterpart of the object from which the drawings were 

 made. To make this matter clear I will mention one or two of the most simple cases. 



If two vertical lines near each other, but at different distances from the spectator, 

 be regarded first with one eye and then with the other, the distance between them 

 when referred to the same plane will appear different ; if the left hand line be nearer 

 to the eyes, the distance as seen by the left eye will be less than the distance as seen 

 by the right eye ; fig. 7- will render this evident ; a d are vertical sections of the two 

 original lines, and h V the plane to which their projections are referred. Now if the 

 two lines be drawn on two pieces of card, at the respective distances at which they 

 appear to each eye, and these cards be afterwards viewed by either of the means above 

 directed, the observer will no longer see two lines on a plane surface, as each card 

 separately shows ; but two lines will appear, one nearer to him than the other, pre- 

 cisely as the original vertical lines themselves. Again, if a straight wire be held be- 

 fore the eyes in such a position 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 dimensions may be accu- 

 rately 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 in- 

 strument which will enable any person to observe all the phenomena 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 adapt- 

 ation of the eye, which enables us to see distinctly at different distances, and which 

 habitually accompanies every different degree of convergence 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 exceed 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 focal 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. 



