We have not hitherto supposed that light is capable of being resolved into elements : and, before Sir I.

Newton discovered that white light is formed by a union of the different coloured rays, and devised a means of decomposing it, the production of the different colours was explained by most absurd suppositions.

During his experiments, however, on the nature of light, Newton found that the different colours are not refracted to the same focus : and that the consequent pro- duction of coloured images was one of the most serious obsta- cles to the perfection of the refracting telescope [100].

It must, at first sight, appear strange that white light should be a com- pound of all the colours : but this may be demonstrated both analytically, thetically, by exper- iment.

If a ray R, fig 226, passing through the aperture R in a window shutter AB, is thrown on a triangular prism P, it will form on a screen DE, placed to receive it, an ob- long coloured image the various tints of which the white ray was composed, being differently refracted, the violet most, and the red least.