important bearing on the proper mode of cutting the stones. The ugly yellowish tint given by the extraordinary ray of sapphire should be avoided by cutting the stone with its table-facet at right angles to the prism edge, which is the direction of single refraction. Whether a ruby should be treated in the same way is a moot point. No doubt if the colour is deep, it is the best plan, because the amount of absorption of light is thereby sensibly reduced, but otherwise the delightful nuances distinguishing ruby are best secured by cutting the table-facet parallel to the direction of single refraction. Yellow corundum also shows distinct dichroism, but by a variation more of the depth than of the tint of the colour; the phenomenon is faint compared with the dichroic effect of a yellow chrysoberyl. The specific gravity also is very constant, varying only from 3·95 to 4·10; sapphire is on the whole lighter than ruby. Corundum has the symbol 9 on Mohs’s scale, but though coming next to diamond it is a very poor second (cf. p. 79). As is usually the case, the application of heat tends to lighten the colour of the stones: those of a pale violet or a yellow colour lose the tint entirely, and the deep violet stones turn a lovely rose colour. On the other hand the action of radium has, as was shown by Bordas, an intensifying action on the colour, and even develops it in a colourless stone. From the latter reaction it may be inferred that often in an apparently colourless stone two or more selective influences are at work which ordinarily neutralize one another, but, being unequally stimulated by the action of radium, they thereupon give rise to colour. The stellate appearance of asterias or star-stones—star-ruby and star-sapphire—results from the regular arrangement either of numerous small channels or of twin-lamellæ in the stone parallel to the six sides of the prisms; light is reflected from the interior in the form of a six-rayed star (p. 38). Some stones from Siam possess a markedly fibrous or silky structure. The synthetical manufacture of ruby, sapphire, and other varieties of corundum has already been described (p. 116). Besides its use in jewellery corundum is on account of its hardness of great service for many other purposes. Small fragments are extensively employed for the bearing parts of the movements of watches, and both the opaque corundum and the impure kind known as emery are in general use for grinding and polishing softer stones, and steel and other metal-work.