2 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
retina, its apparent size may be made to vary with every alteration of the angular 
inclination of the optic axes.” 
I shall resiiine the consideration of the phenomena of binocular vision with this 
subject, because the facts I have ascertained regarding it are necessary to be under- 
stood before entering on the new experiments relating to stereoscopic appearances 
which I intend to bring forward on the present occasion. 
Under the ordinary conditions of vision, when an object is placed at a certain 
distance before the eyes, several concurring circumstances remain constant, and they 
always vary in the same order when the distance of the object is changed. Thus, as 
we approach the object, or as it is brought nearer to us, the magnitude of the picture 
on the retina increases ; the inclination of the optic axes, required to cause the pic- 
tures to fall on corresponding places of the retinae, becomes greater; the divergence 
of the rays of light proceeding from each point of the object, and which determines 
the adaptation of the eyes to distinct vision of that point, increases ; and the dissimi- 
larity of the two pictures projected on the retinae also becomes greater. It is im- 
portant to ascertain in what manner our perception of the magnitude and distance 
of objects depends on these various circumstances, and to inquire which are the 
most, and which the least influential in the judgements we form. To advance this 
inquiry beyond the point to which it has hitherto been brought, it is not sufficient to 
content ourselves with drawing conclusions fiom observations on the circumstances 
under which vision naturally occurs, as preceding writers on this subject mostly have 
done, but it is necessary to have more extended recourse to the methods so successfully 
employed in experimental philosophy, and to endeavour, wherever it be possible, not 
only to analyse the elements of vision, but also to recombine them in unusual manners, 
so that they may be associated under circumstances that never naturally occur. 
The instrument I shall proceed to describe enables these abnormal combinations 
to be made in a very simple and effectual manner. Its principal object is to cause 
the binocular pictures to coincide, with any inclination of the optic axes, while their 
magnitudes on the retinae remain the same; or inversely, while the optic axes remain 
at the same angle, to cause the size of the pictures on the retinae to vary in any 
manner. 
Two plane mirrors inclined 90° to each other are placed together and fixed verti- 
cally upon a horizontal board. Two wooden arms move round a common centre 
situated on this board in the vertical plane which bisects the angle of the mirrors, and 
about 1 j inch beyond their line of junction. Upon each of these arms is placed an 
upright pannel, at right angles thereto, for the purpose of receiving its appropriate 
picture, and each pannel is made to slide to and from the opposite mirror. The eyes 
being placed before the mirrors, the right eye to the right mirror and the left eye to 
the left mirror, and the pannels being adjusted to the same distances, however the 
arms be moved round their centre, the distance of the reflected image of each picture 
from the eye will remain exactly the same, and consequently its retinal magnitude 
