By a triangular piece of glass—a prism,—we are enabled to resolve light into its ultimate rays. The white pencil of light which falls on the first surface of the prism is bent from its path, and coloured bands of different colours are obtained. These bands or rays observe a curious constancy in their positions: the red ray is always the least bent out of the straight path: the yellow class comes next in the order of refrangibility; and the blue are the most diverted from the vertex of the prism. The largest amount of illuminating power exists in the yellow ray, and it diminishes towards either end.[95] It is not uninteresting to observe something like the same variety of colour occurring at each end of the prismatic spectrum. The strict order in which the pure and mixed coloured rays present themselves is as follows:—
1. The extreme red: a ray which can only be discovered when the eye is protected from the glare of the other rays by a cobalt blue glass, is of a crimson character—a mixture of the red and the blue, red predominating.[96]
2. The red: the first ray visible under ordinary circumstances.
3. The orange: red passing into and combining with yellow.
4. The yellow: the most intensely luminous of the rays.
5. The green: the yellow passing into and blending with the blue.
6. The blue: in which the light very rapidly diminishes.
7. The indigo: the dark intensity of blue.
8. The violet: the blue mingled again with the red—blue being in excess.
9. The lavender grey: a neutral tint, produced by the combination of the red, blue, and yellow rays, which is discovered most easily when the spectrum is thrown upon a sheet of turmeric paper.