ect in which the
undulations originate. In like manner a light which we see is referred
to its objective luminous source. But light also and in addition is
reflected from, and thus reveals the presence of the whole body of our
resistant environment. Hence is derived the coloured presentation of
Vision to which the character of extensity attaches. Nothing similar
takes place in the case of the other distantial sensations. If sonorous
undulations excited vibration in every resistant object of the
environment they would undoubtedly come to arrange themselves in an
order resembling the extensity suggested by Vision, though the slower
rate of transmission of sound would detract from the practical
simultaneity in the effect which, as we have seen, largely accounts for
the perception of visual extensity. The universal diffusion of sunlight
is also a determining factor.

* * * * *

The matter becomes still clearer when we contrast the experience of
vident men with what we have been able to learn of the experiences of
the blind. Nowhere have we found this aspect of the question discussed
with the same clearness and ability as by M. Pierre Villey in his
recently published essay, _Le Monde des Aveugles_--Part III.

The blind man, as he remarks, requires representations in order to
command his movements. We must then penetrate the mind of the blind and
ascertain what are his representations. Are they, he asks, muscular
images combined by temporal relations, or are they images of a spatial
order? He replies without hesitation: Both, but, above all, spatial
images. It is clear, he says, that the modalities of the action of the
blind are explained by spatial representations. These must be derived
from touch. What, then, can be the spatial representations which arise
from touch? The blind, he says, are often asked, How do you figure to
yourself such and such an object, a chair, a table, a triangle? M.
Villey quotes Diderot as affirming that the blind cannot imagine.
According to Diderot, images require colour, and colour being totally
wanting to the blind the nature of their imagination was to him
inconceivable. The common opinion, says M. Villey, is entirely with
Diderot. It does not believe that the blind can have images of the
objects around him. The photographic apparatus is awanting and the
photograph cannot therefore be there.

Diderot was a sensationalist. For this school, as Villey remarks,
_