This fact may be demonstrated by throwing a pencil of Sunlight upon one angle of a prism, and receiving the oblong image, so formed, upon a white screen.
The space illuminated and coloured by a pencil of rays analyzed in this way is called "the Solar Spectrum." The action of a prism in decomposing white light will be more fully explained in the next Section. At present we notice only that seven principal colours may be distinguished in the Solar Spectrum, viz. red, orange, yellow, green, blue, indigo, and violet. Sir David Brewster has made observations which lead him to suppose that the primary colours are in reality but three in number, viz. red, yellow, and blue, and that the others are compound, being produced by two or more of these overlapping each other; thus the red and yellow spaces intermingled constitute orange; the yellow and blue spaces, green.
The composition of white light from the seven prismatic colours may be roughly proved by painting them on the face of a wheel, and causing it to rotate rapidly; this blends them together, and a sort of greyish-white is the result. The white is imperfect, because the colours employed cannot possibly be obtained of the proper tints or laid on in the exact proportions.
The decomposition of light is effected in other ways besides that already given:—-
First, by reflection form the surfaces of coloured bodies. All substances throw off rays of light, which impinge upon the retina of the eye and produce the phenomena of vision. Colour is caused by a portion only, and not the whole, of the elementary rays, being projected in this way. Surfaces termed white reflect all the rays; coloured surfaces absorb some and reflect others: thus red substances reflect only red rays, yellow substances, yellow rays, etc, the ray which is reflected in all cases deciding the colour of the substance.
Secondly, light may be decomposed by transmission through media which are transparent to certain rays, but opaque to others.
Ordinary transparent glass allows all the rays constituting white light to pass; but by the addition of certain metallic oxides to it whilst in a state of fusion, its properties are modified, and it becomes coloured. Glass stained by Oxide of Cobalt is permeable only to blue rays. Oxide of Silver imparts a pure yellow tint; Oxide of Gold or Suboxide of Copper a ruby red, etc.
DIVISION OF THE ELEMENTARY RAYS OF WHITE LIGHT INTO LUMINOUS, HEAT-PRODUCING, AND CHEMICAL. RAYS.
The agency of Light produces a variety of distinct effects upon the bodies which surround us. These may be classed together as the properties of light. They are of three kinds—the phenomena of colour and vision, of heat, and of chemical action.