28 INTRODUCTION 



the drawing. It is sometimes well to mark the exact termination 

 of the line by a small cross. 



The name of the drawing in larger letters may be placed at 

 the head or at the foot of the page. The view-point, scale, and 

 date should also be given. Each sheet of the drawings should 

 be initialed for identification, the initials being placed where 

 they cannot be clipped off. 



Actual representation of an object is limited to the two dimen- 

 sions of a plane. The third dimension of solid bodies is ex- 

 pressed by perspective, which is made up of several factors, 

 chiefly (a) binocular vision, (b) aerial perspective, (c) foreshort- 

 ening, (d) sequence of objects, (e) chiaroscuro, or relation of 

 light and shade. 



The part played by binocular vision is of use only in stereo- 

 scopic pictures. Laboratory drawing is limited almost wholly 

 to representation of things as seen by one eye. 



Foreshortening refers to apparent size and shape of objects 

 (or surfaces) as dependent on position and distance. Ordinarily 

 we do not take cognizance of it at all, or only partially. It may 

 be made evident in several ways, as, by inverting the head and 

 looking at things upside down; by looking at them in a mirror 

 and regarding the images as situated on its surface ; or by inter- 

 posing a transparent plane object (wire netting or glass e.g., 

 window-pane) between the eye and the object, perpendicular to 

 the line of vision, and regarding the lines as projected forward 

 and drawn on this surface in one plane. To get a foreshortened 

 drawing it is, then, only necessary to copy on paper the lines as 

 thus seen; or they may be traced with crayon or India ink on 

 the mirror, net, or glass itself. 



Objects that are partly hidden by nearer objects are thereby 

 shown to be more distant than the latter. 



Aerial perspective, affecting color, distinctness of detail, etc., 

 is of importance only when the distances are considerable, as in 

 landscape, and need not be discussed here. 



The shading is self-evident. It is due to the amount of light 

 reflected to the eye from the various surfaces, or areas of a sur- 

 face, which have different directions, and therefore receive and 

 reflect different amounts of light. For the purposes of drawing, 

 the object may be regarded, and represented, as illuminated by 

 light coming from a source above to the left. This simplifies 

 shading and gives it a constant significance in expressing direc- 

 tion of surfaces. (It is to be noted that shading can also be used, 

 as in black and white pictures, to represent color-value or rela- 



