XXX INTRODUCTION 



ieating more than is necessary. The edges of the cover may 

 be cemented as above, but this is not absolutely necessary. 



To mount an object in Canada balsam, it must, after 

 staining, be dehydrated by soaking in alcohol ; then soaked 

 in oil of cloves or other essential oil, or in xylol, benzia, 

 chloroform, ether, etc., tiU the alcohol is removed ; and, lastly, 

 placed in the drop of balsam upon the slide and covered. 



Sections are mounted as follows : — 1. Sections cut frozen 

 are transferred direct to the glycerin on a slide. A thin and 

 delicate section cut frozen cannot well be mounted in balsam, 

 but a moderately thick and tough one may be treated like any 

 other object as described in the previous paragraph. 



2. The methods of mounting in balsam sections cut in 

 paraffin are very numerous. The following answers well, and 

 is especially adapted for mounting sections in series. 



Dry the slide thoroughly at about 60° C. While still warm, 

 •coat it on one side with a delicate layer of shellac — this is 

 done by dipping a glass rod into an alcoholic solution of 

 shellac {see Appendix), and then holding the rod horizontal, 

 laying it flat upon the slide and drawing it evenly over the 

 •surface, when it will leave a thin layer of the solution upon 

 he slide, and the alcohol evaporating immediately leaves an 

 almost imperceptible layer of shellac on the surface. Just 

 before placing the sections upon this, smear it very shghtly 

 with oil of cloves ; lay the sections perfectly flat upon the stUl 

 moist slide, and put the slide for fifteen minutes into an oven 

 at a temperature of 60° C. Remove the slide and pom- 

 turpentine upon it, or immerse the warm slide bodily in the 

 turpentine for a minute or two, to dissolve the paraffin out of 

 the sections ; lift out of the turpentine ; drain about fifteen 

 seconds, drop the balsam upon the sections while they are 

 stUl wet with turpentine ; and cover immediately. 



X. ON MEASURING MICROSCOPIC OBJECTS. 



The eye-piece micrometer is a plate of glass, the upper 



surface of which is either engraved with a scale, or is divided 



into a number of equal squares by cross-lines ruled upon it. 



'This is placed between the lenses of the eye-piece of the 



