ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



767 



three disks provided have 72, 54 and 43 teeth, giving sections ^W> 

 ysVtf and T ^V(T m - respectively. The thickness of the sections is ascer- 

 tained by multiplying the notches in the disk by the number of threads 

 per inch on the spindle. 



Fig. 181. 



Kig. 181a. 



The knife-plate, 2f in. by 4| in., is made of hardened brass polished 

 " dead flat," and has an aperture the same diameter as the tube, but 

 tapers slightly to the top in order to prevent the specimen from turning 



