hould be sought for and carefully
expressed, the outlines being made up of a series of overlappings.

[Sidenote: Shading.]

In Line Drawing shading should only be used to aid the expression of
form. It is not advisable to aim at representing the true tone values.

In direct light it will be observed that a solid object has some
portion of its surface in light, while other portions, those turned away
from the light, are in shadow. Shadows are also cast on the ground and
surrounding objects, called cast shadows. The parts of an object
reflecting the most direct light are called the high lights. If the
object have a shiny surface these lights are clear and distinct; if a
dull surface, soft and diffused. In the case of a very shiny surface,
such as a glazed pot, the light may be reflected so completely that a
picture of the source of light, usually a window, will be seen.

In the diagram on page 95 [Transcribers Note: Diagram V], let A
represent the plan of a cone, B C the opening of a window, and D the eye
of the spectator, and E F G the wall of a room. Light travels in
straight lines from the window, strikes the surface of the cone, and is
reflected to the eye, making the angle of incidence equal to the angle
of reflection, the angle of incidence being that made by the light
striking an object, and the angle of reflection that made by the light
in leaving the surface.

It will be seen that the lines B1D, C2D are the limits of the direct
rays of light that come to the eye from the cone, and that therefore
between points 1 and 2 will be seen the highest light. If the cone have
a perfect reflecting surface, such as a looking-glass has, this would be
all the direct light that would be reflected from the cone to the eye.
But assuming it to have what is called a dull surface, light would be
reflected from other parts also, although not in so great a quantity. If
what is called a dull surface is looked at under a microscope it will be
found to be quite rough, _i.e._ made up of many facets which catch
light at different angles.

[Illustration: Diagram V.

PLAN OF CONE A, LIT BY WINDOW BC; POSITION OF EYE D. ILLUSTRATING
PRINCIPLES OF LIGHT AND SHADE]

Lines B4, C3 represent the extreme limits of light that can be received
by the cone, and therefore at points 3 and 4 the shadow will commence.
The fact that light is reflected to the eye right up to the point 3 does
not upset the theory that it can only be reflected from po