ut "so many inches of black line on a white ground," that
is to say reports a certain _extension_ answering to its
own movement. This quality of extension exists also in our
sound-perceptions, although the explanation is less evident. Notes do not
indeed exist (but only sounding bodies and air-vibrations) in the
space which we call "real" because our eye and our locomotion
coincide in their accounts of it; but notes are experienced, that is
thought and felt, as existing in a sort of imitation space of their own.
This "musical space," as M. Dauriac has rightly called it, has limits
corresponding with those of our power of hearing or reproducing
notes, and a central region corresponding with our habitual
experience of the human voice; and in this "musical space" notes are
experienced as moving up and down and with a centrifugal and
centripetal direction, and also as existing at definite spans or
_intervals_ from one another; all of which probably on account of
presumable muscular adjustments of the inner and auditive
apparatus, as well as obvious sensations in the vocal parts when we
ourselves produce, and often when we merely think of, them. In
visual perception the sweep of the glance, that is the adjustment of
the muscles of the inner eye, the outer eye and of the head, is
susceptible of being either interrupted or continuous like any other
muscular process; and its continuity is what unites the mere
successive sensations of colour and light into a unity of extension,
so that the same successive colour-and-light-sensations can be
experienced either as _one_ extension, or as two or more, according
as the glance is continuous or interrupted; the eye's sweep, when not
excessive, tending to continuity _unless a new direction requires a
new muscular adjustment._ And, except in the case of an
_extension_ exceeding any single movement of eye and head, a new
adjustment answers to what we call _a change of direction.
Extension_ therefore, as we have forestalled with regard to sound,
has various modes, corresponding to something belonging to
ourselves: a _middle,_ answering to the middle not of our field of
vision, since that itself can be raised or lowered by a movement of
the head, but to the middle of our body; and an _above_ and
_below,_ a _right_ and a _left_ referable to our body also, or rather
to the adjustments made by eye and head in the attempt to see our
own extremities; for, as every primer of psychology will teach