146 The Preparation of Microscope Slides 



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 find- 

 ing 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 microt- 

 omist 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 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 a holder that can be 

 inserted into the microtome. Since the majority of sections today are cut 

 on a Spencer rotary microtome, the following description is of the use of 

 one of the holders supplied with this machine, although the ingenuity 

 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. Ill, con- 

 sists of a disc of metal with a roughened surface attached to a cylindrical 

 shank. First of all, this disc must 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 entirely bound by convention, it would be 

 much better for him to obtain a series of small rectangular blocks of some 

 hardwood like maple and to soak these for a day or two in molten wax. 

 After they are removed, drained, and cooled, it is the simplest thing in the 

 world to attach a paraffin block to them and to hold them in the jaws 

 of the microtome. Whether the metal holder or the wooden one is used, 

 the technique is essentially the same. A layer of molten wax is built up 

 on the surface and allowed to cool. The block (see Fig. Ill) is pressed 

 lightly onto this hardened wax and fused to it with the aid of a piece of 

 heated metal. Some people use old scalpels, but the author prefers the 

 homemade brass tool shown in Fig. 111. Care must be taken to press 

 very lightly with the forefinger and to conduct the whole operation as 



