t unless these laws are
attended to. At the present time too little attention is paid to them;
the consequence is that much of the art of the day reflects in a great
measure the monotony of the snap-shot camera, with its everyday and
wearisome commonplace.

IV

PERSPECTIVE OF A POINT, VISUAL RAYS, &C.

We perceive objects by means of the visual rays, which are imaginary
straight lines drawn from the eye to the various points of the thing we
are looking at. As those rays proceed from the pupil of the eye, which
is a circular opening, they form themselves into a cone called the
+Optic Cone+, the base of which increases in proportion to its distance
from the eye, so that the larger the view which we wish to take in, the
farther must we be removed from it. The diameter of the base of this
cone, with the visual rays drawn from each of its extremities to the
eye, form the angle of vision, which is wider or narrower according to
the distance of this diameter.

Now let us suppose a visual ray _EA_ to be directed to some small object
on the floor, say the head of a nail, _A_ (Fig. 17). If we interpose
between this nail and our eye a sheet of glass, _K_, placed vertically
on the floor, we continue to see the nail through the glass, and it is
easily understood that its perspective appearance thereon is the point
_a_, where the visual ray passes through it. If now we trace on the
floor a line _AB_ from the nail to the spot _B_, just under the eye, and
from the point _o_, where this line passes through or under the glass,
we raise a perpendicular _oS_, that perpendicular passes through the
precise point that the visual ray passes through. The line _AB_ traced
on the floor is the horizontal trace of the visual ray, and it will be
seen that the point _a_ is situated on the vertical raised from this
horizontal trace.

[Illustration: Fig. 17.]

V

TRACE AND PROJECTION

If from any line _A_ or _B_ or _C_ (Fig. 18), &c., we drop
perpendiculars from different points of those lines on to a horizontal
plane, the intersections of those verticals with the plane will be on
a line called the horizontal trace or projection of the original line.
We may liken these projections to sun-shadows when the sun is in the
meridian, for it will be remarked that the trace does not represent the
length of the original line, but only so much of it as would be embraced
by the verticals dropped from each end of it, and although line