8 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
distances ; it also shows the angular positions of the camera required to obtain bin- 
ocular pictures which shall appear at a given distance in the stereoscope in their true 
relief. 
Inclination of the optic axes 
2° 
4° 
6° 
S° 
10° 
12° 
14° 
16° 
18° 
20° 
22° 
24° 
26° 
28° 
30° 
Distance in inches 
71-5 
35-7 
23-8 
17-8 
13-2 
11'8 
10-1 
8-8 
7-8 
7-0 
6'4 
5-8 
5-4 
50 
4-6 
The distance is equal to | cotang g; a denoting the distance between the two eyes, 
and 6 the inclination of the optic axes. 
§ 20 . 
As the inclination of the optic axes diminishes by the removal of an object to 
which tliey are directed to a greater distance, not only does the magnitude of the 
pictures projected by it on the retinae proportionately diminish, but the dissimilarity 
of the pictures becomes less. The difference of distance between any two points of each 
of the pictures will diminish until the projections become sensibly similar. Under the 
usual circumstances attending the vision of a solid object placed at a given distance, 
a particular inclination of tlie axes is invariably accompanied by a specific pair of 
dissimilar projections; and if the distance be changed, a different inclination of the 
axes is accompanied by another pair of projections ; but, by means of the stereoscope, 
we have it within our power to associate these circumstances abnormally, and to 
cause any degree of inclination of the axes to coexist with any dissimilarity of the 
two pictures. To ascertain experimentally what takes place under these circumstances 
M. Claudet prepared for me a number of Daguerreotypes of the same bust, taken at 
a variety of different angles, so that I was enabled to place in the stereoscope two 
pictures taken at any angular distance from 2° to 18°, the former corresponding with 
a distance of about 6 feet, and the latter with a distance of about 8 inches*. The 
effect of a pair of near projections seen with a distant convergence of the optic 
axes, is to give an undue elongation to lines joining two unequally distant points, so 
that all the features of a bust appear to be exaggerated in depth. The effect, on the 
contrary, of a pair of distant projections, seen with a near convergence of the axes, 
is to give an undue shortening to the same lines, so that the appearance of a bas-relief 
is obtained from the two projections of the bust. The apparent dimensions in breadth 
and height remain in both cases the same. 
§ 21 . 
To reproduce the conditions of the binocular vision of a solid object as completely 
as possible by means of its two plane projections, it is necessary, as I have before 
stated, that the projections shall be such as correspond exactly with the inclination 
of the optic axes under which they are viewed. I have already shown in § 20 what 
takes place when this condition is not strictly observed, and I may add that the 
