1897.] on Diamonds. 489 



than 3000, wten the work slowly proceeded. Other portions of 

 the stone were found to be comparatively soft, and became harder as 

 the outside was cut away. 



Beautifully white diamonds have been found at Inverel, New 

 South Wales, and from the rich yield of the mine and the white 

 colour of the stones, great things were expected. A parcel of 

 many hundred carats came to England, when it was found they 

 were so hard as to be practically unworkable as gems, and I 

 believe they were ultimately sold for rock-boring purposes. 



I will illustrate the intense hardness of the diamond by an 

 experiment. I place a diamond on the flattened apex of a conical 

 block of steel, and on the diamond I bring down a second cone of 

 steel. With the electric lantern I will project an image of the 

 diamond and steel faces on the screen, and force them together by 

 hydraulic power. You see I can squeeze the stone right into the 

 steel blocks without injuring it in the slightest degree. 



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 diamond 

 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 and coruscations for which the diamond 

 is supreme above all other gems. 



1 hold some of Mr. Streeter's magnificent diamonds in the elec- 

 tric light, and by transmitted light you will see they are black, while 

 by reflected light they fill the room with radiance and colour. 



The following table gives the refractive indices of diamonds and 

 other bodies : — 



Refeactive Indices for the D Line. 



Chromate oflead .. 2 -50-2 -97 



Diamond 2-47-2-75 



Phosphorus .. .. 2-22 



Sulphur 2*12 



Ruby 1-78 



Thallium glass . . 1 ' 75 



Iceland spar .. .. I'Go 



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 



According to Dr. Gladstone, the specific refractive energy — 



