6 PROCEEDINGS OF THE ACADEMY OF 



I use was sorted by eleutriation, a process indispensable before applying it to 

 the production of au acceptable and polish-bearing surface. This method 

 consists in shaking up good flour of emery with water in a tall cylindrical 

 jar, suffering the whole to stand in repose for, say, two seconds, and pouring 

 off into another jar the fluid overlying the sediment. The original deposit 

 may again be treated in the same way, and when the water shall have been 

 added to that first removed, the residual powder is collected, dried and labelled 

 1" emery. 



The finer particles withdrawn in suspension into the second jar are shaken 

 up afresh, allowed to deposit for three seconds, are collected, dried and 

 labelled V emery ; while the finest dust, which subsides very slowly in the 

 decanted water, may be gathered as S^'' or 10''^ emery, the last being fine 

 enough for polishing. 



For most purposes large portions of hard objects destined for microscopic 

 examination are unnecessary ; and besides, the grinding and other prepara- 

 tion of extensive slices involve a useless expenditure of time and patience. 

 Small sections made from fragments or pieces cut off by wheel from a cabinet 

 specimen are altogether satisfactory — indeed, chips flaked off with adroitness 

 are oftentimes extremely useful in directing investigation, and may even bear 

 mounting as permanent objects. » 



As small particles or pieces cannot be readily manipulated in their inde- 

 pendent state, it is necessary to secure them in such manner as to insure the 

 success of their skillful reduction. This may be done by attaching them 

 singly by shellac or hardened balsam to pieces of heavy mirror glass of 

 about an inch in diameter and ground smooth on the edges. Thus fastened 

 a good surface may be obtained for one side. Then the object must be turned 

 over and again cemented by heating the shellac as before, and pressing the 

 prepared surface flat against the glass. Now, as the grinding progresses the 

 qualities of the section become apparent, and the degree of thinness deter- 

 mined by direct inspection of the section under the microscope. 



Large sections, as of shells, can be made with as much precision as smaller 

 ones ; but no one can hope to emulate the late Mr. W. Glen, of Cambridge, 

 unless he have extraordinary skill, coupled with most extensive experience. 



As to the manner of working the lathe and its wheels, 1 feel obliged to re- 

 frain from speaking; for nothing but repeated essays with that instrument, 

 or personal instruction of an adept, can assist the beginner more than the 

 bearing in mind of a few rules of universal application. First^ produce and 

 even, steady and not too rapid motion of the disc ; second, charge the wheel, 

 whether side or edge, with a moderate quantity of emery, or diamond dust, 

 at a time ; third, hold the otiject lightly against the cutting surface ; and 

 fourthly, change the position of the object by rotating it upon its flat surface, 

 and at short intervals, as the grinding progresses. 



When ground and polished the specimen maj' be separated from the shellac 

 and glass by immersion in hot alcohol, and mounted, either by simply fasten- 

 ing it with balsam to a slide, or embalming it in that substance and covering 

 with a thin glass. 



It is worthy of note that many objects which may technically be classed 

 among the hard substances, require for their reduction the intervention of a 

 cutting powder between themselves and the reducing surface, whether wheel 

 or stone. As instances I may mention biliary and other calculi, the dried 

 crystalline lens and phthisical or other calcareous concretions. No hone can 

 smoothly cut these bodies, for its grit soon becomes smoothed out by the 

 adherent debris, and the specimen is either not further att'ected or it is de- 

 stroyed by breaking. The powders I employ are the fine " silex '" of the 

 dentists, and the very fine dust of pumice made even by rubbing while 

 moistened between glass plates. A first surface should be secured by rub- 

 bing the specimen upon a hone made flat by means of emery and water and 

 a flat marble slab, and the coarser and then the finer powder are to be em- 

 ployed and kept moist with water. Now the object must be dried and after- 



