The arch ought to be not less than a foot wide at the bot tom, that the eye at z may have a large field of view through it ; and the eye should be then at least ten inches and a half from the intersection of the threads at r, when the arch is set upright. If the eye be nearer, the boundaries of the view at the sides near the foot of the arch will subtend an angle at z of more than sixty degrees, which will not only strain the eye, but will cause the outermost parts of the drawing to have a disagreeable appearance. To avoid this it will he proper to draw back the sliding bar t till z he fourteen inches and a half distant from r ; then the whole field of view through the foot-wide arch, will not subtend an angle to the eye at z of more than forty-live degrees : which will give a more easy and pleasant view not only of the objects them selves, but of their representations upon the paper on which they are delineated. Hence, whatever may be the width of the arch, the distance of the eye front it should lie in this pro portion : as twelve is to the width of the arch, so is fourteen and a half to the distance of the eye (at 7.) from it.
If a pane of glass, previously coated with thin gum.water, and be fixed in the arch, a person who looks through the hole at it, may delineate upon the glass the objects which lie secs at a distance, and the delineation may lie afterwards transferred to paper. By this means will be saved the trou ble of putting down the arch to take the position of every point, but it will not be so easy to obtain a correct rcpre sentatieni.
PEasrEc•vE OF SIIADOWS.—The shadow of an object is no more than the projection of its contour upon one or more planes, from a given luminous point, and is therefore the dark space upon these planes, occasioned by the intervention of the object, which hides the rays of light from proceeding in straight lines. To avoid difficulties, the luminary, whether the sun or artificial light, is considered as a point, and the sun's rays arc considered as parallel.