We have already mentioned that as a rule such surfaces as are receiving the brightest light (which means they are turned directly towards the source of light), will be the brightest. If we have an object which is rounded in form (such as a cylinder) we will usually find it the lightest in value towards the window. Those portions which are turned away from the light will of course be rather dark. There may be a gradual change of tone from the lightest parts to the darkest, - see Sketch 1, Figure 6, - or if the object has a somewhat irregular surface such as the decagonal prism, shown at Sketch 2 in the same diagram, the values may change gradually plane by plane from the lightest plane to a slighter darker plane, to a still darker one, and so on around to a point on the back opposite the lightest plane. It is not always true, however, that the darkest plane or portion of curved surface will be the one farthest from the source of light, for there is sometimes a certain amount of illumination from some other direction, and even if there is not, there are frequently rays of light reflected onto the parts in shade or shadow, thereby neutralizing the otherwise dark values. Sketches 1 and 3, Figure 6, will serve to illustrate this point (the reader is referred also to Chapter VI (Cast Drawing), Part II. on graded tones). At "1" the brightest value is on that portion of the cylinder receiving the strongest rays of light. Then as the surface curves more and more away from the source of illumination the darker it gets. At "3" a different condition exists. The brightest part of the surface is at "A" as in "1"; then the tone gradually darkens until it reaches "B," which is the darkest. Then at "C" a lighter value is found, caused by the reflection of light rays from some other object.

Figure 6. Modelling or Shading of Objects.

## Planes

Now few objects which we draw have surfaces curving as gradually as those of the cylinder just illustrated or planes so mechanically arranged as those of the prism. • More often the objects are so irregular that the light and shade varies from part to part; there may be many portions turned towards the light and many turned more or less away. These various areas of light and shade which are seen in an object, caused by its irregular form and its position in relation to the source of light, are usually referred to as "planes," even though they do not fully meet the geometric definition of the word.

## Edges Of Planes

In some irregular objects there is quite a definite line of demarkation between the various planes. In gradually curving objects there is no such line - the tone simply grades as we have noted in the case of the cylinder, with no sudden, perceptible change in value. In most objects both of these two conditions exist; in parts the planes seem quite definite, in others they merge together. There is nothing more important than to draw these edges correctly, sharpening them or losing them as the case may be. In the same way there is great difference in the edges of the objects themselves as they come in contrast with the background or with other objects. Some stand out in sharp relief, others are indistinct. Some dark objects become so lost in shadow on their shade side that it is hard to distinguish the form, hence it should not be over-accented in the drawing.