130 



with the varying details of the object-carriers, knife-carriers, 

 etc., make it impossible to attempt here to instruct the stu- 

 dent in the details of microtome manipulation. 



For cutting insect specimens, especially whole bodies with 

 their chitinous outer wall, extreme solidity and rigidity of 

 the microtome in all its parts is absolutely necessary for ac- 

 curate cutting. The heavy, solidly made, sliding micro- 

 tomes of the general character of the Thoma-Jung models 

 are, in our opinion, the microtomes best adapted for general 

 insect work. The cutting is not done so rapidly as by the 

 wheel microtomes, but the sureness is an over-balancing item. 

 Short ribbons, as long as the knife is broad, can be cut with 

 the sliding microtomes and transferred directly to the slide. 



The specimen is cut as a small block out of the larger 

 paraffin block containing it, and mounted on the object-car- 

 rier of the microtome. The block to be cut should be rect- 

 angular in outline, and be set with that margin which will 

 first strike the knife parallel with the knife. All screw ad- 

 justments, such as those holding the knife and the object, 

 should be tightly made, and the block containing the object 

 should be immovably fastened to the object carrier. The 

 sections should be, for general work, from 5 microns to 10 

 microns thick. Except in the case of eggs or delicate tissue, 

 it is rarely necessary to cut thinner than 5 microns. 



The student will undoubtedly meet with more or less 

 trouble in section-cutting. If it is too warm in the labora- 

 tory the sections may stick to the knife ; if too cold, they 

 may roll or break. Rolling of sections is the commonest 

 trouble in cutting. It is often apparently impossible to get 

 that proper temperature or condition which will prevent 

 rolling. There are various expedients for straightening or 

 hindering the rolling of sections that tend to roll. For 

 details and suggestions regarding section-cutting, see Lee's 

 Vade-Mecum, p. 108 et seq. 



