The Mickoscope. 



205 



smooth, free from waves, and so thin as to be only perceptible by the 

 iridescent hue which it imparts to the glass. When ready to make 

 sections, a slide should be selected and a thin coating of clove oil 

 given it with a fine camel's-hair brush over that portion covered with 

 the shellac film. 



Care must be taken not to go over the same spot twice, as the 

 oil dissolves the varnish. 



For cutting embryos, a microtome is almost indispensable ; but 

 fair sections may be made by one used to free-hand cutting with the 

 razor. The labor is much greater, however, and the sections vary 

 greatly in thickness. Before cutting, the paraffin should be 

 trimmed down to within a few lines of the specimen, straight 

 behind and triangular in front (A). Each consecutive section, as 

 removed, must be laid on the 

 prepared slide, beginning at the 

 upper right hand corner and 

 continuing in rows from right to 

 left. Of course, no larger space 

 must be occupied than the size of 

 the cover-glass to be used. 



The sections have a tendency to roll up as they are cut, but this 

 may be prevented by holding the point of a teasing- needle or a 

 camel's-hair brush just over the edge of the knife as it passes 

 through the paraffin ; or, one of the numerous planishers, to be 

 had of dealers in microscopical supplies^ may be used. 



When a sufiicient number of sections have been placed, the 

 slide should be carefully heated over the lamp-flame. This melts 

 the paraffin, and dispels the clove oil, and permanently fixes the 

 sections. By placing the slide on the edge the paraffin may be col- 

 lected to one point. When the slide is cool a few drops of oil of 

 turpentine should be poured over the sections to dissolve the remain- 

 ing paraffin. The length of time during which the turpentine should 

 cover the object depends upon the size of the latter and the amount 

 of paraffin left in the tissues. The oil must then be run off and 

 the glass carefully wiped clean up to the sections. A drop of 

 Canada balsam, and the adjusting of the cover-glass completes the 

 process . 



If " ribbon " sections, as mentioned in § 9, B, are desired, the 

 paraffin block must be trimmed accurately — the sides being parallel 

 and the section- knife parallel to the side. If this is properly done, 

 the edges of the sections will adhere, and a ribbon of almost any length 



