GRINDING AND POLISHING THIN SECTIONS. 195 



141. As there are certain substances, however, the view of whose 

 structure is impaired by mounting in Canada balsam, and which 

 should therefore be mounted either dry or in fluid, a different 

 method of procedure must be adopted with them. If tolerably 

 thin sections of them can be cut in the first instance, or if they 

 are of a size and shape to be held in the hand whilst they are 

 being roughly ground down, there will be no occasion to attach 

 them to glass at all : it is frequently convenient to do this at first, 

 however, for the purpose of obtaining a ' hold ' upon the specimen ; 

 but the surface which has been thus attached must afterwards be 

 completely rubbed away, in order to bring into view a stratum 

 which the Canada balsam shall not have penetrated. As none 

 but substances possessing considerable toughness, such as Bones 

 and Teeth, can be treated in this manner, and as these are the 

 substances which are most quickly reduced by a coarse file and 

 are least liable to be injured by its action, it will be generally 

 found possible to bring the sections to a considerable thinness, by 

 laying them upon a piece of cork or soft wood held in a vice, and 

 operating upon them first with a coarser and then with a finer file. 

 When this cannot safely be carried further, the section must be 

 rubbed down upon that one of the fine stones already mentioned 

 (§ 139) which is found best to suit it : as long as the section is 

 tolerably thick, the finger may be used to press and move it ; but 

 as soon as the finger itself begins to come into contact with the 

 stone, it must be guarded by a flat slice of cork or by a piece of 

 gutta-percha a little larger than the object. Under either of 

 these the section may be rubbed down until it has been reduced 

 to the requisite degree of tenuity ; but even the most careful 

 working on the finest-grained stone will leave its surface covered 

 with scratches, which not only detract from its appearance, but 

 prevent the details of its internal structure from being as readily 

 made out as they can be in a polished section. This polish may 

 be imparted by rubbing the section with putty-powder (peroxide 

 of tin) and water upon a leather strap, made by covering the 

 surface of a board with buff-leather, having three or four thick- 

 nesses of cloth, flannel, or soft leather beneath it : this operation 

 must be performed on both sides of the section, until all the marks 

 of the scratches left by the stone shall have been rubbed out ; when 

 the specimen will be fit for mounting, after having been carefully 

 cleansed from any adhering particles of putty-powder. 



142. Chemical Actions. — One important part of the preparation 

 of Microscopic objects is often effected by the use of Chemical 

 Re-agents. These may be employed either for the sake of removing 

 substances of which it is desired to get rid, in order to bring some- 

 thing else into view, or for the sake of detecting the presence of 

 particular substances in the object under examination. Thus, the 

 Author has found that he has frequently been better able to bring 

 into view particular features in the organization of Forminifera 



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