SHADOW

In Optics, a portion of space from which light is intercepted by an opaque body. As the rays of light proceed in straight lines, every opaque object on which light falls is accompanied with a shadow on the side opposite to the luminous body; and the shadow appears more intense in proportion as the illumination is stronger, because any object placed within it contrasts more strongly with the surrounding objects on which the light is suffered to fall. As every point of a luminous body is a separate focus of illumination, it follows that an opaque object illuminated by the sun, or any other source of light which is not a single though point,not distinguishable must have an from infinite each other number; and of" hence shadows,the shadow of an opaque body received on a plane is always terminated by a penumbra, or partial shadow, the extent of which is greater in proportion to the magnitude of the luminous body, the distance of the opaque body from the plane on which the shadow falls, and the degree of obliqui'.y with which the luminous rays fall on the plane. Shadows are said to be right, or versed, according to the position on the planes on which they are thrown. The shadow of an upright body projected on the plane of the horizon is a right shadow; and that of a body on a vertical plane to which the body is perpendicular, as that of a bar of iron fixed perpendicular in a wall, is a versed shadow. In both cases, the length of the shadow is to the height of the opaque body as the cosine to the sine of the angle which the luminous ray makes with the plane. The ancient geometers availed themselves of this property of shadows to measure the altitudes of objects. Coloured Shadows. — In certain states of the atmosphere the shadows of opaque objects projected on a white surface are frequently observed, about the time of sunrise or sunset, to be of a blue colour. This curious observation appears to have been first made by the celebrated painter Leonardo da Vinci, and is also noticed by Otto Guericke in his Magdeburg Experiments; but no further notice seems to have been taken of the phenomenon until about a century later, when it again occurred to Buffon. Happening to be standing on an eminence about the time of sunset, he perceived the shadows of the trees on a white wall about 30 or 40 feet distant to be coloured with a light green, in dining to blue. The next morning, at sunrise, he repeated the observation; but instead of tinding the shadows green, he found them blue, or rather the colour of lively indigo. Afterwards, he often observed the shadows, both in the morning and evening, but always observed them to be blue; and he remarks that any one may see a blue shadow, if he will hold his finger before a piece of white paper at sunrise or sunset. The phenomenon has since been frequently observed; and it is usually ascribed to the aerial colour of the atmosphere, which enlightens the shadows, and in which the blue rays prevail. (See Pricstly's History of Light, <$-c, 1762.) For remarks on shadows with luminous borders, glorified shadows, and other optical phenomena of the same kind, see Prof. Forbes's Report on Meteorology1840. in the Reports of the British A ociation for Shadow. In Painting, the form which a 1117solid object projects on a surface or surfaces by being interposed be- [s. 1131]