[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 points where the angle of incidence can equal the angle of reflection, as it would seem to do, because the surface being rough presents facets at different angles, from some of which it can be reflected to the eye right up to point 3.  The number of these facets that can so reflect is naturally greatest near the high lights, and gets gradually less as the surface turns more away; until the point is reached where the shadows begin, at which point the surface positively turns away from the light and the reflection of direct light ceases altogether.