NATURAL SCIENCES OF PHILADELPHIA. 



11 



"the surfaces are exposed alternately to the heat of the spirit flame sufficiently 

 to melt the whole balsam. All superfluous balsam now runs out between the 

 principal slide and the paper, and usually bears with it the air-bubbles, so 

 that the section remains perfectly flat, and embedded in the smallest possible 

 quantity of balsam. 



When perfectly cold the upper slide may be removed, the paper readily 

 stripped oS", and the mounting is accomplished. 



To clean off the marginal hard balsam with a hot knife-blade, to clear 

 away residual streaks or border, and to wash to brightness both surfaces of 

 the slides with aqua ammonia on a bit of rag, occupy but a few minutes ; and 

 nothing remains to be done but attach the label duly inscribed. 



There are those who prefer to ornament the edge of the cover with black 

 varnish, which certainly gives a pretty appearance to the slide ; but I would 

 recommend the previous protection of the margin of balsam below it by a 

 thin coat of shellac varnish, which effectually prevents penetration of the 

 black into the balsam, whether soft or in a hardened condition. 



In reviewing this paper, I do not seek to excuse its prolixity, but to express 

 the hope that the methods and processes described in it may obtain the appro- 

 bation of others ; and that they may continue to give the very excellent re- 

 sults which have rewarded my labors, as well as of those friends who have 

 accepted my guidance in microscopical studies. 



This paper was accompanied by thirty-eight preparations for ex- 

 hibition, including sections of teeth, shells and calculi. 



Description of Machine for Grinding Sections of Hard Substances for Micro- 

 scopic Purposes. 

 BY COLEMAN SELLERS. 

 The preparation of thin sections of hard substances, such as bone, teeth, 

 stone, &c., for microscopic purposes is usually attended with more or less 

 trouble, and requires considerable skill on the part of the workman. The 

 chief difficulty is in securing uniformity of thickness in the specimen. A 

 machine lately devised by Mr. J. S. Bancroft, of Philadelphia and in use at 

 the establishment of Messrs. Wm. Sellers & Co., embodies a principle which, 

 applied to this purpose, facilitates the operation. This particular machine is 

 one of those improvements in the direction of enabling unskilled labor to 

 take the place of skilled labor, or to enable skilled workmen to do a greater 

 amount of good work in the same time. It is a well known fact that patient 

 industry combined with mechanical skill will enable marvels of good work- 

 manship to be produced, but those inventions are the most valuable which 

 with the least outlay of capital in costly machinery ena:bles unskilled work- 

 men to produce the same result more expeditiously and if possible more per- 

 fect. The contrivance of Mr. Bancroft was designed to produce a truly plane 

 surface on hardened metals. It consists of a plane metal table, the upper sur- 

 face of which is made accurately true. This plane surface or plate rests on 



hinges at one end and is provided with 

 an adjusting screw under the other end. 

 In the middle of the table is an oblong 

 rectangular slat, say one inch wide, so 

 arranged as to allow the edge of an emery 

 wheel placed below the table to project 

 slightly above the surface of the plate. 

 The emery wheel, supported in suitable 

 bearings, is made to revolve rapidly on 

 its axis, and its periphery becomes an 

 abrading point in the centre of the plane 

 surface of the metal plate. By means of 

 the regulating screw the plate can be so 



adjusted as to determine the distance 



'n"!"'i':"!"[iWl that the edge of wheel shall project above 



