The different refrangibility of the rays of light is demonstrated by the prism. If a beam of light from the sun be let into a darkened room, and be received upon a white screen or opposite wall, it will form a circular image, and will be of one uniform whiteness. If a prism be interposed, so that the light must pas' through it before it reaches the wall, the image is no longer circular or white ; it assumes an oblong shape, terminated by semicircular arches, and exhibits seven different colours. This oblong image is called the spectrum. In the whole range of philosophical experiment, a more beautiful appearance cannot be presented to the eye, and instructive nature will appear not less extra ordinary than its beauty, when. it is considered, that the investigation of the cause of it led Sir Isaac Newton to form the first rational theory of the cause of colours. The seven colours of the spectrum are called the original, or pri mary colours. If a spectrum be divided into 100 parts, the red part of it is found to occupy 11 of these parts; the orange 8, the yellow 14, the green 17, the blue 17, the indigo 11, and the violet 22. The red part of the spectrum is nearest the prism ; and the violet, at the greatest distance. It is clear, from this, that light is not homogenous, because the attractive power of the prism is greater upon some parts of it than upon other parts. Accordingly, it is gene rally concluded that the solar beam or white light is composed of particles differing in size and density ; that this difference of their size and density is the cause of their being differently refrangible ; and that the separation of the rays of one or more sizes from the rest, by various means, produces all the diver sity of colours which affect our sight. It is found, that the red part of light is capable of struggling through thick and resisting mediums, when all the other colours are stopped. Thus, the sun appears red when seen through a fog.
The particles which compose orange light are next, in size and refrangibility, to the red ; and so on to the violet, which consists of the smallest particles, and which are, therefore, the moat turned out of their course. White is composed of all the primary colours, mixed together in their due proportions. When bodies reflect the rays of light in the proportion in which they exist in the solar beam, they appear white ; when they reflect none of the rays, they appear black.
Convex lenses in their simple state have been applied to collect the heat of the sun's rays, for purposes similar to those of burning mirrors. A burning lens must be convex ; a burning mirror, concave ; became both produce their effect by concentrating into a focus the rays of light and heat incident upon a large surface. As the rays which pass through a convex lens, or are reflected from a concave mirror, are united at its focus, their effect is so much the greater, as the surface of the lens or mirror exceeds that of the focus. Thus, if a lens four inches in diameter collect the sun's rays into a fbcus at the dis tance of one foot, the focal image will not be more than one-tenth of an inch broad. The surface of this circle is 1600 curies less than the surface of the lens, consequently the density of the sun's rays within it is proportionately increased. It has been found, that large lenses and mirrors burn with irresis tible intensity when properly constructed, dispersing the hardest metals and other substances into gas, often in a few seconds. See Buammo Gtass, and the various othei_optical instruments, under their respective names.