64 MATERIALS AND PROCESSES OF SLIDE-MAKING 



manufacturer to be reground or to avoid that portion of the blade containing 

 the nick when cutting sections. It must be emphasized that the only purpose 

 of setting is to produce a cutting facet and that grinding, which cannot be 

 done in the ordinary laboratory, is required for the removal of knife imper- 

 fections. The next question is that of "stropping" the blade of the knife by 

 pulling it backward across a leather surface in the manner shown in Fig. 36. 

 If the knife has been set properly, stropping, the only purpose of which is to 

 polish the facet, is quite unnecessary. The nature of the leather surface which 

 is used for stropping makes it obviously impossible to pull the knife blade 

 forward, and there is a grave risk in pulling it backward that the facet, instead 

 of becoming polished on its flat surfaces, will become rounded on its edges 

 and thus undo the work of setting. Certainly no beginner should be permitted 

 to use a strop until he has demonstrated his ability to set a knife-edge to the 

 point where it will cut an excellent section without stropping. It is also 

 strongly recommended to the beginner that he examine the edge of a knife 

 under the low power of a microscope before setting, after setting, and 



after stropping. 



Mounting the Block. After the knife has been sharpened and the micro- 

 tome has been selected, the block is trimmed to the correct shape and attached 

 to the object holder of the microtome. The rough block of wax containing 

 the object first must be removed from whatever was used to cast it in or, if a 

 paper box was used, the box cut away roughly with a knife. The block should 

 now be held against a light so that the outlines of the contained object can 

 be seen clearly. The block is trimmed until the object lies in the center of a 

 perfect rectangle with the major axis of the object exactly parallel to the long 

 sides. This is best achieved by finding the major axis at right angles to which 

 the sections are to be cut and by trimming down one side of the block with 

 a sharp safety-razor blade, taking off only a little wax at a time. If one tries 

 to remove a large quantity of wax, there is the danger of cracking the block. 

 After one side is 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, and it is essen- 

 tial 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 diffi- 

 culty, 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 essential only 

 that they be parallel with each other. At this stage plenty of wax should be 

 left both in front of and behind the object. 



This trimmed block now has to be attached to some holder which can be 



