90 REV. J. CLARK MURRAY : À PROBLEM IN VISUAL PERCEPTION. 
the visual illusion we are considering. It is popularly supposed, and even the older 
psychologists seemed to assume, that touch reveals to us the absolute dimensions of bodies, 
and corrects in this way the illusory appearances of magnitude that are presented by sight. 
This view, however, was long ago exposed by Berkeley, and has been thoroughly 
dispelled by the more accurate examination of tactile perceptions inaugurated in the 
experiments of E. H. Weber. It is now known that the tactual, as well as the visual, 
perception of dimension is based on the associations of experience. 
In consequence of this, the tangible magnitude of a body is not an unvarying quantity ; 
on the contrary, it depends on the part of the organism with which the body is in contact. 
On some parts of the skin the points of a pair of compasses are felt to be distinct only 
when they are placed between twoand three inches apart, while they can be 
distinguished by the tip of the forefinger when separated only by one-twelfth of an inch, 
and even at the half of that distance by the tip of the tongue. As a result of this, two 
fixed points appear to be more distant when felt by a sensitive than when felt by an 
obtuse part of the skin. If the two poinis, therefore, are drawn from the soft part of the 
arm over the palm tothe fingertips, they appear to separate, while they seem to approach 
if drawn in the opposite direction. Consequently, a body impresses us as being of greater 
magnitude when touched by a more acute part of the organism. A familiar illustration 
of this is the fact, that a tooth, when touched by the tongue, appears larger than when 
touched by the finger. Weshould commonly express this by saying that the tooth appears 
larger than it really is; for our ideas of real magnitude are connected mainly with the 
special organ of touch, the fingertips. 
Have we not in this the reason of the illusion which makes objects seem unexpectedly 
large to a congenitally blind man, when first restored to sight ? Our ideas of magnitude 
depend on the extent of sensitive surface which seems to be affected by an object. Now, 
that extent is to be measured, not by its real dimension, but by the acuteness of the sensi- 
bility with which it is endowed. But this acuteness itself is proportional to the minute 
subdivision of the ultimate elements which form the essential organ of sensibility. For 
this reason, to touch itself a body seems to cover a larger expanse at a part of the organism, 
where the papillae and the corpuscula tactus and the tactile nerves are distributed in more 
refined minuteness and in greater multiplicity. Now, it is not necessary to institute an 
exact commensuration of the ultimate elements of organic sensibility in the hand and the 
eye respectively ; it is sufficient to dwell upon the obvious fact, that the retina exhibits a 
structure adapted for a much more minute delicacy of sensation than the acutest part of 
the skin. Accordingly when a person, who has been accustomed to form his ideas 
of magnitude from the impression of objects on the skin, is suddenly made to feel them 
affecting a much more acute organ, it is not surprising that he should “see everything 
much larger than he had supposed from the idea obtained by the sense of touch.” 
The additional fact seems curious, that Cheselden’s patient, after the second eye was 
couched, saw objects magnified with it, even when he used the eye first cured at the same. 
time. But this is only in accordance with the familiar fact, that many other illusions, 
which arise from natural causes, do not cease even after their illusory character has been 
exposed by scientific analysis. 
