ROYAL INSTITUT1014.
On the 23d instant, Mr. N. S. Maskelyne delivered a lecture "On Diamonds." Among the characters of the diamond, hardness and lustre were conspicuous, the former rendering it useful for a variety of me- chanical purposes, the most important being that of cutting glass. The glazier's diamond was a rounded crystal, producing a smooth, deep cut in the glass, and this crisp-like form was the only one applicable to the purpose. The writing diamond was a different instrument, consist- ing of a fragment turned to a fine point, and producing only a shallow scratching impression. The simple apparatus by which diamond cutting was effected was then exhibited, and its mode of motion explained to the audience. The diamond owed its great value for ornamental purposes to its lustre, and this was due to an extraordinary refractive power by which the light striking on the several facets of the jewel was in a great measure prevented passing through its substance, and consequently was reflected. To prove this, the lecturer interposed a diamond between the electric light and a screen, when, to the surprise of the audience, a deep shadow was thrown, with a small central spot of light transmitted through the middle or "table" of the crystal ; at the same time, the ceiling and walls of the lecture-theatre were dotted with light, reflected from the different facets. The lecturer then examined the several hy- potheses suggested for the formation of diamonds ; none of them, he be- lieved, were satisfactory, but the fact might be significant, that diamonds were always found in rocks in which silicate predominated. After bringing forward evidence of the probable identity of the Koh-i-noor with the famous diamond described in the Sultan Babers' autobiography, and whose history could be traced back to very ancient times, the lec- turer exhibited a magnificent collection of diamonds, kindly lent for the purpose by Mr. Martyn, and their splendours were vividly displayed under the powerful beams of the electric light.
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