GRINDING AND POLISHING THIN SECTIONS. 209 



so resinous as to be brittle. If it be too soft, as indicated by its 

 too ready yielding to the thumb nail, it should be boiled a little 

 more; if too hard, which will be shown by its chipping, it should 

 be re-melted and diluted with more fluid balsam, and then set 

 aside to cool as before. When it is found to be of the right con- 

 sistence, the section should be laid upon its surface, with the 

 polished side downwards ; the slip of glass is next to be gradu- 

 ally warmed until the balsam is softened, special care being taken 

 to avoid the formation of bubbles ; and the section is then to be 

 gently pressed down upon the liquefied balsam, the pressure 

 being at first applied rather on one side than over its whole area, 

 so as to drive the superfluous balsam in a sort of wave towards 

 the other side, and an equable pressure being finally made over 

 the whole. If this be carefully done, even a very large section 

 may be attached to glass, without the intervention of any air- 

 bubbles ; if, however, they should present themselves, and they 

 cannot be expelled by increasing the pressure over the part be- 

 neath 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. 



110. 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 oft' the chief part of it at once by the slitting-wheel. So 

 soon, however, as it approaches the thinness of a piece of ordi- 

 nary card, it should be rubbed down with water on one of the 

 smooth stones previously named, the glass slip being held be- 

 neath 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 Microscope (particular regard being had to the precaution 

 specified in 86), 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 fore-finger 

 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, how- 

 ever, 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 fre- 



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