160 CAMERA OBSCURA.—SHADOWS.

All the luminous points visible in the heavens during abeautiful night; all the objects which make up a delightfullandscape, when seen on a clear and serene day, are paint-ed in our eye, in all their due proportions and forms,their colours and their flitting shadows, by means of cones,in the manner just described.

Camera obscura .—Art imitates nature, constructing achamber similar to the interior of the eye, allowing thelight only to enter by a glass or lens, similar to the pupilof the eye, S, fig. 22, pi. 9- The light forms on thesides of this chamber, as on the retina, abed, a representa-tion of the colours, forms, and movements of the objectsthat are in the front of the lens. If we receive this trans-mitted light on paper, we can draw the outline which itforms, and reproduce the same colours, the same lightsand shades.

The rays, which emanate from a single point S, fig. 20,pi. 2, and meet an opaque surface abedef, cannot proceedonwards ; the rays which touch the outline of this surfaceproceeding onwards, separate the space beyond the opaquebody into two parts ; one illuminated, and the other de-prived of light by it. The space deprived of light is call-ed the shadow of the opaque body. Thus, when anopaque surface or body is placed before a luminous point,its shadow is limited by a conical surface, having the lu-minous point for its vertex.

Silhouettes .—'This property of the luminous rays hasbeen turned to account in drawing portraits on a planesurface, similar to any given profiles. The profile tobe imitated, abode, fig. 20, is placed in a plane parallelto that on which the portrait is to be drawn. A light,such as a candle or lamp, at a convenient distance, be-comes the vertex of a cone, having for its base theprofile to be copied. The cone is continued to the planeof the portrait, so as to trace on it another base, ABCD...similar to the former, and marked by the outline whichis the limit to the shadow of the profile; this base is thesilhouette of the profile.