But it is not the hardness of the diamond so much as its optical qualities that make it so highly prized. It is one of the most refracting substances in nature, and it also has the highest reflecting properties. In the cutting of diamonds advantage is taken of these qualities. When cut as a brilliant the facets on the lower side are inclined so that light falls on them at an angle of 24° 13´, at which angle all the incident light is totally reflected. A well-cut brilliant should appear opaque by transmitted light except at a small spot in the middle where the table and culet are opposite. All the light falling on the front of the stone is reflected from the facets, and the light passing into the diamond is reflected from the interior surfaces and refracted into colours when it passes out into the air, giving rise to the lightnings, the effulgence, and coruscations for which the diamond is supreme above all other gems. The following table gives the refractive indices of diamonds and other bodies: REFRACTIVE INDICES FOR THE D LINE Chromate of lead 2·50-2·97 Diamond 2·47-2·75 Phosphorus 2·22 Sulphur 2·12 Ruby 1·78 Thallium glass 1·75 Iceland spar 1·65 Topaz 1·61 Beryl 1·60 Emerald 1·59 Flint glass 1·58 Quartz 1·55 Canada balsam 1·53 Crown glass 1·53 Fluor-spar 1·44 Ice 1·31 In vain I have searched for a liquid of the same refraction as diamond. Such a liquid would be invaluable to the merchant, as on immersing a stone the clear body would absolutely disappear, leaving in all their ugliness the flaws and black specks so frequently seen even in the best stones. THE DIAMOND AND POLARISED LIGHT Having no double refraction, the diamond should not act on polarised light. But as is well known, if a transparent body which does not so act is submitted to strain of an irregular character it becomes doubly refracting, and in the polariscope reveals the existence of the strain by brilliant colours arranged in a more or less defined pattern, according to the state of tension in which the crystal exists. I have examined many hundred diamond crystals under polarised light, and with few exceptions the colours show how great is the strain to which some of them are exposed. On rotating the polariser, the black cross most frequently seen revolves round a particular point in the inside of the crystal; on examining this point with a high power we sometimes see a slight flaw, more rarely a minute cavity. The cavity is filled with gas at enormous pressure, and the strain is set up in the stone by the effort of the gas to escape. I have already said that the great Cullinan diamond by this means revealed a state of considerable internal stress and strain.