MR. WHEATSTONE ON THE PHYSIOLOGV OF VISION. 
the optic axes are parallel, and when they converge greatly, although the pictures 
remain, in both cases, at the same distance from the eyes. The a(la|)tation is, there- 
fore, not entirely dependent on the divergence of the rays of liglit which proceed 
from the object regarded, but also, in some degree, on the inclination of the optic 
axes. I have acquired by practice considerable power of adjustment, or rather dis- 
adjustment, of the eyes, and can, without having recourse to artificial means, see 
the binocular picture distinctly when its perceived magnitude is widely different. 
Those to whom such an effort is painful may employ short-sighted spectacles to see 
the binocular picture when the eyes converge within the limit of distinct vision for 
the distance at which the pictures are placed ; and long-sighted spectacles when the 
eyes converge beyond that limit, or become parallel. 
There is a means of avoiding to a very considerable extent the influence of the 
adjustment of the eyes, and thereby enabling the pictures to be seen distinctly within 
the entire range of the inclination of the optic axes. This is by looking at the 
reflected images in the mirrors through two very minute apertures, not larger than 
fine pin-holes, placed near each eye, and illuminating the pictures by a very strong 
light; sunshine in the middle of the day answers the purpose very well. By this 
expedient the divergence of the rays of light is greatly diminished, and the adapta- 
tion of the eyes does not materially influence the result. 
§ 18. 
Leaving this subject, I will now revert to the stereoscope and its effects. 
Since 1838 numerous modifications of the stereoscope have occurred to me, and 
several ingenious arrangements have also been proposed by Sir David Brewster and 
Prof. Dove ; but there is no form of the instrument which has so many advantages 
for investigating the phenomena of binocular vision as the original reflecting stereo- 
scope. Pictures of any size may be placed in it, and it admits of every kind of 
adjustment. 
I have constructed a very portable reflecting stereoscope which is represented at 
fig. 3. The sides fold over the mirrors, and the mirrors then fold into a box, which 
is not larger than 6 inches in any of its dimensions. To avoid the second feeble 
reflection from the anterior surface of the silvered glass, which has a bad effect when 
the attention is attracted to it, I have sometimes employed reflecting prisms. The 
reflecting surfaces of the prisms should be silvered in order to obviate the unequal 
brightness of the field of view on each side of the limit of total reflection ; and as it 
would be too costly to employ very large prisms, they should have an adjustment 
to accommodate their distance to the width between the eyes of the observer. 
I have, for many years past, employed also another means to occasion, without 
any straining of the eyes, the coincidence of the pictures so that the image in relief 
shall appear of the same magnitude and at the same distance as the object which 
they represent would do if it were itself directly regarded. In this apparatus, prisms 
