4 
MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
nitude ; the magnitude of the picture on the retina, the retinal magnitude ; and the 
magnitude we estimate the object to be from its retinal magnitude and the inclina- 
tion of the optic axes conjointly, I name the perceived magnitude. 1 do not use the 
term apparent magnitude, because, according to its ordinary acceptation, it sometimen 
means what I call retinal, and at other times what I name perceived magnitude. 
We have seen in what manner our perception of magnitude is modified by the new 
associations which this instrument enables us to form ; let us now examine how our 
perception of distance is affected by them. If we continue to observe the binocular 
picture whilst it apparently increases or decreases, in consequence of the inclination 
of the optic axes varying while the magnitude of the impressions on the retinae 
remains the same, it does not appear either to approach or to recede ; and yet if we 
attentively regard it in any fixed position, it is perceived to be at a different distance. 
On the other hand, if we continue to regard the binocular picture, enlarging and 
diminishing in consequence of the change of retinal magnitude while the convergence 
of the axes remains the same, we perceive it to approach or recede in the most 
evident manner ; but on fixing the attention to it, when it is stationary, at any 
instant, it appears to be at the same distance at one time as it is at another. 
Convergence of the optic axes therefore suggests fixed distance to the mind ; vari- 
ation of retinal magnitude suggests change of distance. We may, as I have above 
shown, perceive an object approach or recede without appearing to change its 
distance, and an object to be at a different distance, without appearing to approach 
or recede ; these paradoxical effects render it ditficillt, until the phenomena are well 
apprehended, to know, or to express, what we actually do perceive. 
It is the prevalent opinion that the sensation which accompanies the inclination of 
the optic axes immediately suggests distance, and that the perceived magnitude of 
an object is a judgement arising from our consciousness of its distance and of the 
magnitude of its picture on the retina. From the experiments I have brought 
forward, it rather appears to me that what the sensation which is connected with the 
convergence of the axes immediately suggests is a correction of the retinal magnitude 
to make it agree with the real magnitude of the object, and that distance, instead of 
being a siniple perception, is a judgement arising from a comparison of the retinal 
and perceived magnitudes. However this may be, unless other signs accompany 
this sensation the notion of distance we thence derive is uncertain and obscure, 
whereas the perception of the change of magnitude it occasions is obvious and un- 
mistakeable. 
To see, in their full extent, the variations of magnitude exhibited by the instru- 
ment I have described, it is necessary to attend to the following observations. 
As the inclination of the optic axes corresponding to a different distance is habi- 
tually, under ordinary circumstances, accompanied with the particular adaptation of 
the eyes required for distinct vision at that distance, it is difficult to disassociate 
these two conditions so as to see with equal distinctness the binocular picture when 
