MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
3 
will be unchanged. But as the two reflected images do not occupy the same place 
when the pictures are in different positions, to cause the former to coincide the optic 
axes must converge differently. When the arms are in the same straight line, the 
images coincide while the optic axes are parallel ; and as they form a less angle with 
each other, the optic axes converge more to occasion the coincidence. When the 
arms remain in the same positions, while the pannels slide towards or from the 
mirrors, the convergence of the optic axes remains the same, but the magnitude of 
the pictures on the retinae increases as the distance decreases. By the arrangement 
described, and which is represented by figs. 1 and 2 Plate I., the reflected pictures 
are always perpendicular to the optic axes, and the corresponding points of the pic- 
tures, when they are exactly similar, fall upon corresponding points of the retinae. 
The instrument has an adjustment for otherwise inclining them if it be required. 
Let us now attend to the effects produced. The pictures being fixed at the same 
distance from the mirrors, there is a certain adjustment of the arms at which the 
binocular image will appear of its natural size, that is, the size we judge the picture 
itself to be when we look at it directly; in this case the magnitude of the pictures 
on the retinae and the inclination of the optic axes preserve their usual relation to 
each other. If now the arms be moved back, so as to cause a less convergence of 
the axes, the image will appear to increase in magnitude until the arms are in a 
straight line and the optic axes are parallel ; and, on the other hand, if the arms be 
moved forwards, so as to form a less angle, the optic axes will converge more, and 
the image will appear gradually smaller. In this manner, while the retinal magnitude 
remains the same, the perceived magnitude of the binocular object varies through a 
very considerable range. 
The instrument being again adjusted so that the image shall be seen of its natural 
size ; on sliding the pictures nearer the mirrors its perceived magnitude will be 
augmented, and on sliding them from the mirrors it will appear diminished in size. 
During these variations of magnitude the inclination of the optic axes remains the 
same. 
The perceived magnitude of an object, therefore, diminishes as the inclination of 
the axes becomes greater, while the distance remains the same ; and it increases, 
when the inclination of the axes remains the same, while the distance diminishes. 
When both these conditions vary inversely, as they do in ordinary vision when the 
distance of an object changes, the perceived magnitude remains the same*. 
Before I proceed further it will be proper to explain the meaning of some of the 
terms I employ. I call the magnitude of the object itself, the real or objective mag- 
* Several cases of the alteration of the perceived magnitude of objects are mentioned by Dr. R.. Smith 
(Complete System of Opticks, 1738, vol. ii. p. 388, and rem. 526 and 532) ; and Dr. R. Darwin (Philosophical 
Transactions, vol. Ixxvi. p. 313) observed that when an ocular spectrum was impressed on both eyes it appeared 
magnified when they were directed to a wall at a considerable distance. The facts noticed by these authors 
are satisfactorily explained by the above considerations. 
B 2 
