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.
10. The fluorescent rays: which are either a _pure silvery blue_ or a delicate _green_.
Newton regarded the spectrum as consisting of seven colours of definite and unvarying refrangibility. Brewster and others appear to have detected a great diffusion of the colours over the spectrum, and regard white light as consisting only of three rays, which in the prismatic images overlap each other; and from these--red, yellow, and blue--all the others can be formed by combination in varying proportions. The truth will probably be found to be, that the ordinary prismatic spectrum is a compound of two spectra:--that is, as we have the ordinary rainbow, and a supplementary bow, the colours of which are inverted, so the extraordinary may be somewhat masked by the intense light of the ordinary spectrum; and yet by overlapping produce the variations of colour in the rays. We have already examined the heating power found in these coloured bands, which, although shown to be in a remarkable manner in constant agreement with the colour of a particular ray, is not directly connected with it; that is, not as the effect of a cause, or the contrary. The chemical action of the solar rays, to which from its important bearings we shall devote a separate chapter, has, in like manner with heat, been confounded with the sun’s luminous power; but although associated with light and heat, and modified by their presence, it must be distinguished from them.