194 SECTIONS OF HARD SUBSTANCES. 



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 section 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 Micro- 

 scope (particular regard being had to the precaution specified in 

 § 117), 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 thinness 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 frequent 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 per- 

 fection 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^rs^ 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 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 (§ 157) : this is particularly the case with Fossils to which a 

 deep hue has been given by the infiltration of some colouring matter, 

 and with any substances whose particles have a molecular aggre- 

 gation that is rather amorphous than crystalline. When a suffi- 

 cient 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 (§ 161). 





