MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
9 
mind is not unpleasantly affected by a considerable incongruity in this respect ; on 
the contrary, the effect in many cases seems heightened l>y viewing the sofid appear- 
ance, intended for a determinate degree of inclination of the axes, under an angle 
several degrees less ; the reality is as it were exaggerated. When tlie optic axes are 
parallel, in strictness there should be no difference between the pictures presented to 
each eye, and in this case there would he no binocular relief ; but I find that an ex- 
cellent effect is produced when the axes are nearly parallel by pictures taken at an 
inclination of 7° or 8°, and even a difference of 16° or 17° has no decidedly had 
effect. 
This circumstance enables us to combine the ideal amplification arising from view- 
ing pictures placed near the eyes under a small inclination, or even parallelism, of 
the optic axes mentioned in 17, with the perception of solidity arising from the 
dissimilarity of the projections ; for this purpose, the pictures in the refracting stereo- 
scope, or their reflected images in the reflecting instrument, must be viewed through 
lenses the focal distance of which is equal to the distance between them and the 
pictures ; the perceived magnitude of the binocular image will increase with the 
nearness of the pictures, and depends almost entirely on the disassociation of the 
retinal magnitude from its usually accompanying inclination of the optic axes, the 
actual magnifying power of the lenses having a very small influence. 
The sole use of the lenses is to render the rays of light parallel, which it is neces- 
sary they should be for distinct vision when the optic axes are parallel. When the 
reflecting stereoscope is employed, this means of magnifying the effect is not of much 
utility, as pictures of any size may be adapted to that instrument. But in the case 
of the refracting stereoscope it may be advantageously made use of. By combining 
lenses with the refracting stereoscope, described in § 18, Daguerreotypes somewhat 
wider than the width between the eyes may be employed. Sir David Brewster has 
used, to effect the same purpose, semi-lenses with their edges directed towards each 
other, which serve at the same time to render the rays less convergent and slightly 
to displace the pictures towards each other. Two corresponding Daguerreotypes, 
each not exceeding in breadth the width between the eyes, being placed close to 
each other, and viewed with lenses of short focal distance, will even without the aid 
of the prisms give an apparently highly magnified binocular image in bold relief. 
There is a peculiarity in such images worthy of remark ; although the optic axes 
are parallel, or nearly so, the image does not appear to be referred to the distance 
we should, from this circumstance, suppose it to be, but it is perceived to be much 
nearer, and indeed more so, as the pictures are nearer the eyes, though the inclination 
of the optic axes remains the same, and should therefore suggest the same distance ; 
it seems as if the dissimilarity of the projections, corresponding as they do to a nearer 
distance than that which would be suggested by the former circumstance alone, 
alters in some degree the perception of distance. 
I recommend, as a convenient arrangement of a refracting stereoscope for viewing 
MDCCCLII. c 
