PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 199 



out the intervention of any air-bubbles; if, however, they should present 

 themselves, and they cannot be expelled by increasing the pressure over 

 the part beneath which they are, or by slightly shifting the section from 

 side to side, it is better to take the section entirely off, to melt a little 

 fresh balsam upon the glass, and then to lay the section upon it as 

 before. 



194. When the section has been thus secured to the glass, and the 

 attached part thoroughly saturated (if it be porous) with hard Canada 

 balsam, it may be readily reduced in thickness, either by grinding or 

 filing, as before, or, if the thickness be excessive, by taking off the chief 

 part of it at once by the slitting wheel. So soon, however, as it 

 approaches the thinness of a piece of ordinary card, it should be rubbed 

 down with water on one of the smooth' stones previously named, the 

 glass slip being held beneath the fingers with its face downwards, and the 

 pressure being applied with such equality that the thickness of the sec- 

 tion shall be (as nearly as can be discerned) equal over its entire surface. 

 As soon as it begins to be translucent, it should be placed under the 

 Microscope (particular regard being had to the precaution specified in 

 143), and note taken of any inequality; and then, when it is again laid 

 upon the stone, such inequality may be brought down by making special 

 pressure with the forefinger upon the part of the slide above it. When the 

 thinness of the section is such as to cause the water to spread around 

 it between the glass and the stone, an excess of thickness on either side 

 may often be detected by noticing the smaller distance to which the 

 liquid extends. In proportion as the substance attached to the glass is 

 ground away, the superfluous balsam which may have exuded around it 

 will be brought into contact with the stone; and this should be removed 

 with a knife, care being taken, however, that a margin be still left round 

 the edge of the section. As the section approaches the degree of thin- 

 ness which is most suitable for the display of its organization, great care 

 must be taken that the grinding process be not carried too far; and fre- 

 quent recourse should be had to the Microscope, which it is convenient 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 preparations, 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 advisable to stop short as soon as a good section has been, 

 made, and to lay it aside ' letting well alone' whilst the attempt 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 consumed the first section, however, still 

 remaining: whereas, if the first, like every subsequent section, be sacri- 

 ficed in the attempt to obtain perfection, no trace will be left <( to show 

 what once has been." In judging of the appearance of a section in this 

 stage under the* Microscope, it is to be remembered that its transparence 

 will subsequently be considerably increased by mounting in Canada 

 balsam: 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 sub- 

 stances whose particles have a molecular aggregation that is rather 

 amorphous than crystalline. When a sufficient thinness has been at- 

 tained, the section may generally be mounted in Canada balsam; and the 

 mode in which this must be managed will be detailed hereafter ( 210). 



