508 Prof. Wheatstone on the Physiology of Vision. 



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 pic- 

 ture, 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 ; variation 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 difficult, 

 until the phaenomena are well apprehended, to know, or to ex- 

 press, what we actually do perceive. 



It is the prevalent opinion that the sensation which accom- 

 panies 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 experi- 

 ments 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 magni- 

 tude to make it agree with the real magnitude of the object ; and 

 that distance, instead of being a simple perception, is a judge- 

 ment arising from a comparison of the retinal and perceived 

 magnitudes. However this may be, unless other signs accom- 

 pany 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 unmistakeable. 



To see, in their full extent, the variations of magnitude exhi- 

 bited by the instrument I have described, it is necessary to attend 

 to the following observations. 



As the inclination of the optic axes corresponding to a differ- 

 ent distance is habitually, under ordinary circumstances, accom- 

 panied 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 bin- 

 ocular picture when the optic axes are parallel, and when they 

 converge greatly, although the pictures remain, in both cases, at 



