WM. WEBB ON "nOBEUT's TESTS." 157 



is SO great that I am tempted to state as fully as I am able the 

 reasons of my obstinate tenacity. 



If it were possible by fluoric acid, or by other means, to procure 

 a division from side to side — that is to say, across the middle of the 

 bands of one of Nobert's Tests, the vertical section of the first band 

 would present this appearance — 



No. 5. 



And this is all clear enough — the vertical section would test the 

 Test. Applying this vertical test to the fine bands, quite another 

 state of things will be found to exist. In this last specimen No. 5 

 has the surface untouched, except by each separate incision. I now 

 advisedly adopt the word incision, for the word line applies no 

 more to these diamond cuttings than it does to the Suez Canal. 

 If the incisions were to be filled with black lead or other opaque 

 substance, the surfaces would become palpable lines. 



The first few bands would present the same state of things, main- 

 taining the same clearly defined incisions, with intervening surface 

 spaces, the optical effects of which T pass by for the present moment. 

 Upon proceeding beyond the first few bands, and arriving at the fine 

 bands, the vertical section would present the appearance of engraving 



No. 6. 



which is caused by the tool making contact thus, and moving 



No. fi. 



laterally a less distance than the extreme width of the incision, 

 almost entirely annihilating the one side of each of the extreme or 

 end incisions of the bands. Each end incision having unequal sides 

 is most easily proved by focussing for heigh th and depth with a 

 moderately high power ; but, when I come to the intervening in- 

 cisions, the matter is complicated by other phenomena. To illustrate 

 this clearly I have prepared the grossly exaggerated specimen No. 

 8. At this point of the investigation I cannot lay too much stress 



L 2 



