falling drops of rain. It was impossible to overlook the resemblance of the ordinary rainbow to the comparatively rare lunar rainbow, to the bow which appears upon the spray of a waterfall, or even upon beads of dew suspended on grass and spiders’ webs. In all these cases the uniform conditions are rays of light and round drops of water. Roger Bacon had noticed these conditions, as well as the analogy of the rainbow colours to those produced by crystals.1 But the knowledge was empirical until Descartes and Newton showed how the phenomena were connected with facts concerning the refraction of light.
There can be no better instance of an empirical truth than that detected by Newton concerning the high refractive powers of combustible substances. Newton’s chemical notions were almost as vague as those prevalent in his day, but he observed that certain “fat, sulphureous, unctuous bodies,” as he calls them, such as camphor, oils spirit of turpentine, amber, &c., have refractive powers two or three times greater than might be anticipated from their densities.2 The enormous refractive index of diamond, led him with great sagacity to regard this substance as of the same unctuous or inflammable nature, so that he may be regarded as predicting the combustibility of the diamond, afterwards demonstrated by the Florentine Academicians in 1694. Brewster having entered into a long investigation of the refractive powers of different substances, confirmed Newton’s assertions, and found that the three elementary combustible substances, diamond, phosphorus, and sulphur, have, in comparison with their densities, by far the highest known refractive indices,3 and there are only a few substances, such as chromate of lead or glass of antimony, which exceed them in absolute power of refraction. The oils and hydrocarbons generally possess excessive indices. But all this knowledge remains to the present day purely empirical, no connection having been pointed out between this coincidence of inflammability and high refractive power, with other laws of chemistry or optics. It is worth notice, as pointed out by Brewster, that if Newton had argued concerning two minerals, Greenockite and Octahedrite, as he did concerning diamond, his predictions would have proved false, showing sufficiently that he