242 PEOCEEDINGS OF SOCIETIES. 



in the next number of the Joiirnal, He directed the attention of the 

 Fellows to one species of great beauty, which had been named 

 Aulacodiscus superhus. The paper will be foimd at page 205. 



The thanks of the meeting were voted to Mr. Kitton for his com- 

 munication. 



The President called upon Mr. Wenham to say a few words to 

 the meeting uj^on the microscopical effects produced upon glass by the 

 sand-blast process, which was exhibited at the recent meeting of the 

 British Association. 



Mr. F. H. Wenham said that before doing so he should like to 

 refer to a matter which was under discussion at the last meeting of 

 the Society with reference to the question whether the " eye spot " in 

 Coscinodiscus could really be the effect of a perforation. It would no 

 doubt be remembered that the President was of opinion that no image 

 would be seen unless formed upon something as a screen, and that he 

 had himself undertaken to test the matter by experiment during the 

 vacation. He had done so in a variety of ways, and had come to the 

 conclusion that a real circular perforation, either in an opaque or semi- 

 opaque body, was an unmistakable thing. 



The President suggested that a dark spot might in some cases be 

 produced by diffraction. 



Mr. Wenham made the following remarks on the appearance of 

 the American sand-blast process under the microscope : — 



The pattern shown on this piece of glass was produced by the 

 American " sand-blast " process, in a few seconds. As the appearance 

 of the " greyed " surface under the microscope is quite distinct from 

 that of ordinary ground glass, I bring it before the notice of this 

 Society, as the microscope gives us some insight into the modus 

 operandi. It was stated at the late meeting of the British Association, 

 in the discussion that followed the description of the process, that 

 a large crystal of corundum was speedily perforated with ordinary 

 sea-sand and a blast pressure of 300 lbs. per square inch. Corundum 

 is several degrees beyond emery in hardness, approaching near to that 

 of diamond. But it was further stated that under the conditions 

 named diamond itself speedily became worn away. At first sight it 

 appears extraordinary that the hardest known material should quickly 

 be destroyed by one infinitely softer. The microscope indicates that 

 this is caused by the force and velocity of impact ; it is not a grinding 

 process at all, but a battering action, similar to that of leaden bullets 

 against a block of granite. 



A polished glass surface exposed for an instant to the sand-blast 

 shows an aggregation of points of impact, from which scales of fractured 

 glass have broken away in an irregular radial direction. It ajjpears as 

 if a pellet of glass had been driven in by the collision of the sand, and 

 the wedge-like action thus set up had driven away the surrounding 

 glass. All these spots or indentations, when tested by the polariscope, 

 show a coloured halo round each, proving that the glass surface is 

 under strain and ready to yield to further fracture. 



The action, therefore, is not so much due to the hardness of the 

 striking particles as the force and velocity of impact. This is sufii- 



