210 PREPARATION OF OBJECTS. 



quent recourse should be had to the Microscope, which it is con- 

 venient to have always at hand, when work of this kind is being 

 carried on. There are many substances whose intimate structure 

 can only be displayed in its highest perfection, when a very little 

 more reduction would destroy the section altogether ; and every 

 Microscopist who has occupied himself in making such prepara- 

 tions, can tell of the number which he has sacrificed in order to 

 attain this perfection. Hence if the amount of material be limited, 

 it is a good rule to stop short as soon as a good section has been 

 made, and to lay it aside "letting well alone" whilst the at- 

 tempt is being made to procure a better one ; if this should fail, 

 another attempt may be made, and so on, until either success 

 has been attained, or the whole of the material has been con- 

 sumed, the first section, however, still remaining : whereas, if 

 the first, like every successive section, be sacrificed in the attempt 

 to obtain perfection, no trace will be left to " show what has 

 been." In judging of the appearance of sections in this stage 

 under the Microscope, it is to be remembered that its transpa- 

 rence will subsequently be considerably increased by mounting 

 in Canada balsam ( 125) ; this is particularly the case with fossils, 

 to which a deep hue has been given by the infiltration of some 

 coloring matter ; and with any substances whose particles have a 

 molecular aggregation, that is rather amorphous than crystalline. 

 When a sufficient thinness has been attained, the section may 

 generally be "mounted" in Canada balsam; and the mode in 

 which this must be managed, will be detailed hereafter ( 129). 

 111. 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 dif- 

 ferent 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 ; is 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 at- 

 tached, 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 considera- 

 ble 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 ( 109), 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 



