Every object under the influence of a single light receives it only upon that surface which is exposed to its direct rays. No better example of this can be given than the effect of the sun upon the earth, half of which is at all times enveloped in darkness, modified only by reflections from other bodies. The angle of reflection is the same as the angle of incidence; that surface therefore will appear brightest to the eye which forms an equal angle between it and the light.
From this the light gradually decreases until it merges into the shadow; this is termed the tone of half-tint. The broad shadow is modified towards its extremities by partial light, reflected from surrounding objects; this may be called the reflection. Between the reflection and half-tint a small portion, unaffected either by direct or reflected light, remains in total darkness; this is termed the depth.
There is another shadow which is cast from an object in light upon the surface beneath it, to this is given the name of cast-shadow, characterized by its flatness and cutting edges. This comprises the whole principle of light and shade, whose invariable laws may be observed, more or less modified according to circumstances, in every object in nature.
The modifications to which they are subject are owing to the opacity of the medium through which objects are seen and to reflected lights.
Density of atmosphere renders distant objects less distinct; therefore those lights and shades which are nearest to the eye will be the strongest. For the same reason a body will appear in half-tint towards its extreme edges, near which the high-light and depth cannot be.
A round body, if of the same color as the background, will appear darker at its extreme edges; if lighter than the background, will be lost in it. Some rub in the shade with a stump and powdered chalk, touching up the darkest parts with the point; others use the point and rub it down with the stump or finger or a piece of rough paper; others shade with the point from beginning to end; which latter mode is preferable for beginners, as it is cleaner and demands a greater degree of care and knowledge. There may be other methods and modifications or mixtures of these which can be used as they best suit the object to be represented.