230 Mr. N. S. Maskelytie, [March 23, 



which it previously held, while traversing the air. It results from 

 this, that the critical angle — the angle beyond which all light internally 

 incident on the surface is totally reflected internally, and whose sine is 

 the reciprocal of the refractive index,^ — is in the diamond so small an 

 angle as 26° ; a fact which enables us to cut diamonds for the pur- 

 poses of jewellery, so as to produce an effect which, in any other gem, 

 would only be produced by stones of much greater thickness. Thus, 

 the diamond-cutter, by the instinct of an art which worked out prac- 

 tically this problem at a time when science had not determined " the 

 law of sines," takes advantage of this principle in cutting the diamond 

 into the brilliant form. 



The brilliant is fundamentally a double pyramid or cone, truncated 

 by a large plane called the table, on one end, and by a small one called 

 the culet, at the other. The adjustment of the angles of the two 

 pyramids has to be so made, that the girdle of the stone presents a pris- 

 matic edge, the angle of which must be as much as twice the critical 

 angle, or as nearly so as the original form of the diamond permits. 

 This angle, which in the diamond is 48^, would require to be 68^ to 70^ 

 for the sapphire or chrysoberyl, and above 80^ for glass. The adjust- 

 ment of the relative sizes of the table and the culet are also very 

 important; and the object aimed at in both these adjustments is that 

 no light shall penetrate the diamond from above, that shall not be 

 totally reflected internally, and so thrown back again through the 

 summit planes, called the bezil planes, or the table. Meeting these 

 planes at various inclinations, the light is shattered into a thousand 

 hues, by the dispersive power of the stone. [A companative experiment 

 was exhibited, by which the spectrum formed by a flint glass prism 

 was brought into comparison with one formed by the table and one of 

 the inferior planes of a thin brilliant.] 



The geographical distribution and geological association of the 

 diamond have not as yet thrown much light on its origin. In India, 

 Malacca, Borneo, in Brazil, Mexico, the gold states of the United 

 States, and in the Urals it is found in beds of rivers or alluvial deposits. 

 In Australia and in Algiers it is reported to have been found, and 

 under similar conditions. In Brazil it has been traced to its rock- 

 home in the itacolumite (a micaceous quartzose schist, often containing 

 talcose minerals, and intersected by quartz veins), and also in a horn- 

 blende slate continuous with the itacolumite. But whether these are 

 its parent rocks, or whether — as they are probably metamorphic in 

 their nature — its origin dates from an earlier state of the materials 

 that have become transmuted by time and the play of chemical and 

 physical forces into itacolumite and hornblende slate, we are not in a 

 position to declare. The companions of the diamond do not tell its 

 history in a much less vague language. Gold seems in every diamond 

 country to be either an associate or the not distant neighbour of the 

 diamonds. Tourmaline, chrysoberyl, chrysolite, topaz, kyanite, oxides 

 of titanium and of iron, quartz as jasper, and in other forms, are 

 frequently found with them. 



