66 MATERIALS AND PROCESSES OF SLIDE-MAKING 



edge is parallel, lowering the block again, and then comparing the relation of 

 the upper edge with the edge of the razor. 



Cutting Paraffin Ribbons. The first step in cutting sections on this type 

 of microtome is to make sure that every one of the set screws seen in Fig. 38 

 is fully tight. The set screws holding the block holder may be tightened in 

 any order, provided that the result leaves the block correctly orientated, but 

 those connected with the knife must be done in the correct order. First the 

 knife is inserted into the holder and fixed firmly but not tightly in place by 

 the two bearings at each end. The tightening of these screws causes the two 

 movable holding arms to hold the knife near its edge. The knife is held' in a 

 pair of hemicylinders which may be moved to adjust the knife angle (see Fig. 

 34D). The knife angle should be set at that which experience has shown to 

 be desirable — no guide other than experience can be used— and the two set 

 screws which lock these inclinable hemicylinders in place then tightened. 

 While the knife is held in place, the two original set screws should be screwed 

 as tightly as the thumb can bear. This leaves the two set screws which come 

 through the inclinable hemicylinders and bear on the back of the knife. These 

 two set screws should be tightened simultaneously and uniformly. The effect 

 of this is to force the knife upward and thus wedge it with extreme firmness 

 in the knife holder. 



After all is tight, the handle on the microtome is turned until the block is as 

 far back as possible, and the entire knife moved on its carriage until the edge of 

 the blade is about a quarter of an inch in front of the block. A last minute check 

 is now made to make sure that the divisions of the setting device exactly coincide 

 with the thickness desired. The handle is rotated rapidly until the block starts 

 cutting. The front face will rarely be parallel to the blade of the knife, so that 

 a considerable number of sections will have to be cut until the entire width of 

 the block is coming against the knife. No particular attention need be paid to 

 the quality of this initial ribbon, which may be thrown away. 



If all is not going well and the ribbon is not coming off in a perfect con- 

 dition, refer to Table 1. The remaining operations of preparing and mounting 

 the ribbon are seen far more clearly in illustration than by description. As soon 

 as the ribbon is the width of the knife in length, a dry soft brush, held in 

 the left hand, is slipped under the ribbon, which is then raised in the manner 

 shown in Fig. 38. Care should be taken that a few sections always remain in 

 contact with the blade of the knife because, if the ribbon is lifted till only the 

 edge of the section lies on the edge of the knife, the ribbon will break almost 

 invariably. As the handle is turned, the brush in the left hand is moved away 

 until the ribbon is the length of whatever sheet of paper one has to receive 



