MAKING SECTIONS 61 



Choice of a Microtome. Microtomes may be broadly divided into two 

 classes. In the first of these the block remains stationary, while the knife is 

 moved past it. In the second group are those in which the block moves past a 

 stationary knife. The first class (Fig. 32) is commonly known as a Schantz 

 microtome (after the original German model). They are made by several man- 

 ufacturers, and are rarely used for the preparation of serial sections. They have 

 the advantage that relatively large blocks may be cut but they have the dis- 

 advantage that no ribbon can be obtained broader than the width of the knife. 



A Spencer or rotary microtome is shown in Fig. 33. In this type of micro- 

 tome, the rotation of the large wheel causes the block holder to move verti- 

 cally up and down in most instances through a distance of about three inches. 

 The portion which slides up and down has, at the end opposite the block, a 

 rectangular plate of hardened steel inclined at an angle of about 45 degrees. 

 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 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 

 which 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 sec- 

 tion 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 microns, the pawl will not engage the ratchet perfectly but will chip off a 

 small portion of brass at each revolution. It requires only a few weeks opera- 

 tion 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 the mechanism of it 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 com- 

 plex a microtome is used; if the knife-edge is not perfect, there is no chance 

 of securing a perfect section. Ordinary razors are not satisfactory for the pro- 

 duction of fine sections. It is necessary to secure a microtome knife, preferably 

 from the manufacturer of the microtome. Another type of microtome knife 

 employs the edge of a safety-razor blade in a special holder; this does not, in 

 the author's opinion, give as good results as a solid blade. 



