Making Sections 149 



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 % mm 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 2. The remaining operations of preparing and 

 mounting the ribbon are seen far more clearly in illustration than by de- 

 scription. 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. 112. Care should be taken that a 

 jew sections always remain in contact with the blade of the knife because, 

 i f the ribbon is lifted till only the edge of the section lies on the edge of 

 t he 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 it on. Legal-size 

 (foolscap) paper is employed quite commonly and is shown in Fig. 113. 

 Notice that the left-hand edge of the ribbon has been laid flat some dis- 

 tance from the edge of the paper and that a loop large enough to avoid 

 strain on the ribbon attached to the knife is retained with the brush, 

 while the ribbon is cut with a rocking motion of an ordinary scalpel or 

 cartilage knife. The larger and colder this scalpel is, the less likelihood 

 there is of the section adhering to it. The purpose of leaving a good margin 

 around the edge of the paper is that it may be desirable to interrupt rib- 

 bon cutting for a time and to continue later. In this case the worker 

 should furnish himself with a little glass-topped frame, which is laid over 

 the paper to prevent the sections from being blown about. As the inex- 

 perienced worker will soon find out, the least draft of air, particularly 

 the explosive draft occasioned by someone opening the door, is sufficient 

 to scatter the ribbons all over the room. These operations of carrying the 

 ribbon out with the left hand, transferring the brush to the right hand, 

 and cutting the ribbon off are continued until the whole of the required 

 portion of the block has been cut and lies on the paper. 



The ribbon must be divided into suitable lengths for mounting on a 

 slide (Fig. 114). Although in theory a section should be of the same size 

 as the block from which it came, this practically never occurs in practice 



