parts juxtaposed in space.
Bach point of the retina might send to the brain a detached
impression; these might be comparable, but not necessarily in their
spatial position. The ear sends to the brain such a manifold of
impressions (since the ear also has an apparatus by which various
external differences in rapidity of vibrations are distributed into
different parts of the organ). But this discriminated manifold is a
manifold of pitches, not of positions. How does it happen that the
manifold conveyed by the optic nerve appears in consciousness as
spatial, and that the relation between its elements is seen as a
relation of position?

An answer to this question has been suggested by various
psychologists. The eye, by an instinctive movement, turns so as to
bring every impression upon that point of the retina, near its centre,
which has the acutest sensibility. A series of muscular sensations
therefore always follows upon the conspicuous excitement of any
outlying point. The object, as the eye brings it to the centre of
vision, excites a series of points upon the retina; and the local sign,
or peculiar quality of sensation, proper to each of these spots, is
associated with that series of muscular feelings involved in turning
the eyes. These feelings henceforth revive together; it is enough
that a point in the periphery of the retina should receive a ray, for
the mind to feel, together with that impression, the suggestion of a
motion, and of the line of points that lies between the excited point
and the centre of vision. A network of associations is thus formed,
whereby the sensation of each retinal point is connected with all
the others in a manner which is that of points in a plane. Every
visible point becomes thus a point in a field, and has a felt
radiation of lines of possible motion about it. Our notion of visual
space has this origin, since the manifold of retinal impressions is
distributed in a manner which serves as the type and exemplar of
what we mean by a surface.

_Values of geometrical figures._

Sec. 21. The reader will perhaps pardon these details and the strain
they put on his attention, when he perceives how much they help
us to understand the value of forms. The sense, then, of the
position of any point consists in the tensions in the eye, that not
only tends to bring that point to the centre of vision, but feels the
suggestion of all the other points which are related to the given one
in the web of