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nesses of lines are necessary to enable it to be properly read and understood. These lines are generally of three degrees of thickness, and are defined as fine, medium, and shadow or thick lines. Their use, however, without some well-defined rule of application, would be futile ; as the very reverse effect of that intended would be produced by the incorrect use of either of them. 34. Now, as the proper application of these lines is directly con- cerned with the effect caused by light falling on an object, it is a matter of importance in this special kind of drawing that a uniform 77 78 FIRST PRINCIPLES OF rule be adopted with respect to the direction in which the light is supposed to fall upon the object represented. With the free-hand draughtsman or artist, this direction is optional, as he can adapt it to the way he thinks most conducive to effect in showing up any particu- lar object in his picture ; but with the mechanical draughtsman, as his drawings are not representations of objects as they appear to the eye, but are projections obtained by parallel rays from all parts of them falling upon certain planes having definite relations to each other, but represented by his sheet of paper, he has to adopt some rule of illu- minating the visible surfaces of his objects, in accordance with the system he uses in projecting their outlines. Although the illuminant is the sun, and its light is diffused equally around, it is generally assumed that we see objects by light coming from above and behind us ; but it is evident that if the light shone directly from behind, the spectator would be in his own light, and part of the object would be in shade. The light must then be assumed to come