112 



THE ART OF MAKING MICROSCOPE SLIDES Mounting block 



the beginner that he should examine the 

 edge of a knife under the low power of a 

 microscope before setting, after setting, 

 and after stropping. 



Mounting the Block 



The knife being sharpened and the 

 microtome selected, it now remains to 

 trim the block to the correct shape and to 

 attach it to the object holder of the micro- 

 tome. The rough block of wax containing 

 the object must be first removed from the 

 mold or, if a paper box was used, the box 



and it is essential that these should be 

 exactly parallel to each other. A skilled 

 microtomist can cut these edges parallel 

 with a safety-razor blade without very 

 much difficulty, but numerous devices 

 have been described from time to time 

 in the literature to enable one to do this 

 mechanically. It does not matter if these 

 two edges are exactly parallel with the 

 plane of the object; it is only essential 

 that they be parallel with each other. At 

 this stage plenty of wax should be left 

 both in front of, and behind, the object. 



Fig. 63. Stropping a microtome knife. 



cut away roughly with a knife. The block 

 should now be held against a hght so that 

 the outlines of the contained object can 

 be clearly seen. The block is then trimmed 

 until the object lies in the center of a per- 

 fect rectangle, with the major axis of the 

 object exactly parallel to the long sides. 

 This is best achieved by finding first the 

 major axis, at right angles to which the 

 sections are to be cut, and trimming down 

 one side of the block with a sharp safetj'- 

 razor blade, taking off only a little wax 

 at a time. If one tries to remove a large 

 quantity of wax there is danger of crack- 

 ing the block. When one side has been 

 shaved to a flat surface, the other side is 

 shaved parallel to it. The top and bottom 

 surfaces of the block may now be shaved. 



This trimmed block has now to be at- 

 tached to some holder which can itself be 

 inserted into the microtome. Since the 

 majority of sections today are cut on a 

 Spencer rotary microtome, we will de- 

 scribe the use of one of the holders sup- 

 phed with this machine, though the in- 

 genuity of man has not yet succeeded in 

 devising a worse method of attaching a 

 paraffin block to a microtome. The holder, 

 which is seen in Fig. 64, consists of a disk 

 of metal with a roughened surface at- 

 tached to a cyhndrical shank. This disk 

 must first of all be covered with a layer of 

 wax and it is extraordinarily difficult to 

 get wax to adhere to these chromium- 

 plated surfaces. If the worker is not en- 

 tirely bound by convention, it would be 



\ 



