126 THE MIcROSCOPE. 
THE KNIFE. 
The knife should be ground so that the under surfaces of 
its edge and back are strictly parallel. The blade should 
vary from five to eight inches in length and should be of the 
best tempered steel, in order not only that it may be capable of 
receiving an edge, but also that it may retain the edge. The 
blade should be either biconcave or at least have one of its sur- 
faces slightly concave. A knife appears to work the best when 
its lower surface is slightly concave and its upper surface more 
concave still. 
METHOD OF USING. 
When ready for work only about the quantity of embed- 
ding mixture required should be melted. The mixture should 
be heated just enough to melt it and no more, and a water 
bath is best for this purpose. If too much heat be applied the 
paraffin will shrink from the sides of the well of the micro- 
tome when cooling, and, also, the high degree of heat might 
affect the tissue to be embedded. 
The tissue is now removed from the alcohol and placed on 
a piece of blotting paper, which will absorb the excess of the 
alcohol. The surface of the tissue rapidly dries by the evapo- 
ration of the spirit, and when the surface only is dry the whole 
specimen is dipped into a weak solution of gum-arabic. But 
few realize the value of this simple step, yet it saves a great 
amount of trouble and annoyance. The tissue is removed 
from the gum solution and the surface allowed to become quite 
dry. It is then dipped in the melted paraffin and immediately 
withdrawn. The layer of paraffin will speedily harden and will 
protect the tissue from the more prolonged heat of the embed- 
ding mixture when placed in the microtome. 
The tissue thus prepared is placed in the well of the micro- 
tome, close to that edee of the tube which is situated nearest 
the edge of the knife as it rests on the glass plate ready for 
cutting. The melted paraffin is slowly added until the tissue is 
completely covered. As the paraffin shrinks in cooling it is 
best to have plenty of it over the top of the specimen that it 
may not become again uncovered. 
