Making Sections 141 



A Spencer or rotary microtome is shown in Fig. 105. In this type of 

 microtome, the rotation of the large wheel causes the block holder to 

 move vertically up and down in most instances through a distance of 

 about 3 in. The portion that slides up and down has, at the end opposite 

 the block, a rectangular plate of hardened steel inclined at an angle of 

 about 45°. Under the pressure of a powerful spring, this plate bears 

 against a hardened steel knob, which is itself connected to a micrometer 

 screw. As the handle is rotated, a pawl works against a ratchet to move the 

 micrometer screw and thus the knob connected with it through a given 



Fig. 105. American Optical Company rotary microtome. 



distance for each rotation. As the knob moves forward, it bears on the 

 diagonal plate, which moves the block the required distance forward at 

 each revolution. This mechanism is very costly to make and is liable to 

 have a large number of minor defects that are not always apparent until 

 one has started section cutting. One of the most important things to watch 

 is that the knob which controls the section thickness is so moved that an 

 exact number of microns is indicated. If, for example, the knob is moved 

 so that the indicator line lies between 9 and 10 /x, the pawl will not engage 

 the ratchet perfectly but will chip off a small portion of brass at each revo- 

 lution. It requires only a few weeks of operation under these careless 

 conditions to destroy the ratchet wheel, which will have to be replaced at 

 the factory. No inexperienced student should ever be trusted with one 

 of these machines until its mechanism has been explained and clearly 

 demonstrated to him. 



Knives and Knife Sharpening. The most important single factor in the 

 production of good sections is the knife used in cutting. It does not matter 

 how much care has been taken in the preparation of the block or how 



