LECTURE  XI

§ I. The Separation of Visual Ideas ; Influence of Boundary
Lines. § II. Ide.as of Depth. § III. Relations between
Apparent Magnitude and Distance of Objects.

§  I

WE have now shown at some length how the mind comes
to arrange visual impressions in spatial order upon a
plane surface. But our knowledge of the formation of the field
of vision has given us no idea either of the nature of external
objects, or of the visible parts of our own body. The impres¬
sions, though spatially disposed, have not as yet been brought
into those relations in virtue of which they are arranged as
separate ideas, each apprehended as a whole of definite spatial
form. How does this separation come about ? How do we
pass from the spatial perception which leaves its objects side by
side without distinction or difference to the idea of objects
which are spatially separate ?

First of all, it is plainly the boundary lines of objects which
separate them from one another, and further divide up a single
object into parts. They afford a definite resting-place to the
fixating eye. Whenever we have a series of objects suddenly
presented to us, the eye is arrested by the sharpest boundary
lines. It thus learns the rough outlines of objects first, and
from these passes by degrees to the finer delineations of their
parts. This influence of boundary lines on the movement and
fixation of the eye may be easily proved by experiment. We
hang, e.g., before a white surface a number of vertical black
threads of different diameter, and allow an observer to look
through a tube towards the screen in such a way that the threads
lie in his field of vision. Supposing that the observer knows
nothing  beforehand  of the  arrangement  and  nature  of   the