262 PREPARATION OF SECTIONS 



arranged over the bottom crust blow upon the surface of the 

 paraffin to harden it the more quickly, and plunge the tray 

 into cold water as soon as the surface crust will bear this. The 

 more c^uickly the paraffin is cooled the more firmly it sets about 

 the material. The material may be left thus imbedded in paraffin 

 until it is needed for sectioning. 



Sectioning Material Imbedded in Paraffin. — Tear off the 

 paper tray and with a knife score deeply around the piece of 

 desired material on both top and bottom surfaces, and then 

 break the piece out. This will be called the paraffin block. 

 Melt a piece of paraffin on the surface of the object carrier 

 of the microtome. In the microtome shown in Fig. 143 the 

 object carrier may be simply a piece of pine wood about a centi- 

 meter in cross-section which is to be clamped firmly in the 

 jaws of the microtome. Before the melted paraffin on the 

 object carrier has time to harden press into it the paraffin block, 

 setting it up in the position to give sections in the desired direc- 

 tion; then pass a hot needle around the base of the block so 

 as to fuse it thoroughly with the paraffin bed and make 

 a firm union. Pare the sides of the paraffin block so that the 

 opposing faces are parallel, and adjust the object carrier on 

 the microtome so that the knife, standing at right angles to 

 its line of motion, will have its cutting edge parallel with the 

 face of the block turned toward it. Now the sections may 

 be cut and they should adhere and form a ribbon. In cutting 

 paraffin sections the knife does not need to be wet with alco- 

 hol or anything else as in other cases. If the paraffin breaks 

 away from the material as the sections are cut the infiltration 

 may not have been successful, or the temperature of the room 

 may be too low. If the sections crumple up as they are cut 

 the room is probably too warm. The ribbons ought to be 

 straight, and if the front and back faces of the paraffin block 

 are trimmed parallel they are pretty sure to be straight. Sec- 

 tions seldom need to be cut thinner than .005 mm., and .010 

 mm. is a suitable thickness for most purposes. In micrometry 



