Author Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1856 Excerpt:...angle may extend either from below upwards, or from the right towards the left, or from a distant point towards us, we must select the most suitable of these courses for giving us distinct shadows. As we are generally accustomed to sec objects illuminated by light coming from above, it is natural that we should determine on employing rays coming from above downwards. Further, as our drawings are intended to represent the visible surfaces of objects, we must suppose those sides that are exposed to us, illuminated; and therefore select rays coming from behind us, and striking on and past the objects. As to whether these rays shall come in a direction from left towards right, or from right towards left, is a matter of indifference. The only circumstance which can affect our choice in this last respect is, that draughtsmen generally sit with their left side towards the light, and therefore are more likely to select rays coming in the direction to which they are accustomed: that is, from left towards right. Now, in order to see clearly the effect of rays supposed to come in the chosen direction, in illuminating certain surfaces, and in producing shades and shadows, we shall suppose a piece of paper, cut to the form of Fig. 42, consisting of three equal squares, and that portions of the diagonals of these squares were drawn, lying at mitre-angles, or angles of 45, with the sides of the squares. Now, let us suppose that the paper is folded Fig. 43. f Fig. 42. along the two lines a b and a c, so as to turn up the two squares perpendicular to the third, and to each other, as in Fig. 43, forming three sides of a square box, with the diagonal lines drawn upon them. If the lines of projection upwards, towards the right and towards the front from the extremities of thes...