462 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



For fastening the knife a thumb-screw, 0, fig. 391, serves; hut 

 in the modified form of the instrument designed by the Zoological 

 Station, Naples, this is replaced by a single head-screw. C, fig. 392, 

 which is provided with holes and tightened by means of a lever; 

 and to give greater freedom to the iise of the knife there are several 

 holes drilled and tapped into which this screw fits. 



The knives of the form A, fig. 391, are generally screwed directly 

 to the knife-carriage, and are used for cutting very large sections, the 

 oblique position shown in the figure being the one that is generally 

 indicated for the cutting of very large objects. This knife is now 

 seldom used except in pathological observations and in studies on the 

 central nervous svstem. 



PIG. 392. The Thoma microtome with the usual zoologist's knife. 



knife, however, is also made upon another model, E, fig. 392 ; 

 it then has a, special holder , in which it is secured in a conical slit 

 by the screws ft, ft 1 , and firmly held. 



For deep objects requiring considerable length to cut from, there 

 are plates provided for elevating the knives and the knife-holders. 



The knife-holder shown in fig. 392 can be rotated round the axis 

 formed by the screw c. This allows of any degree of slant or 

 obliquity of direction being given to the knife, from the str ctly 

 i raiis\ersal position shown in fig. 392 up to and beyond the slanting 

 position shown in fig. 391. But it provides no means of altering 

 the till of the blade, that is, of elevating or depressing the back of 

 the blade relativelv to its edge a point of considerable importance, 

 to which we shall return later on. To meet this difficulty, the 

 maker (It. . I iing. 12 Landhausstrasse, Heidelberg ; his instruments, 

 as well as price lists, may be obtained from Mr. C. Baker, 244 High 

 Holhorn. London) supplies wedges to be inserted under the knife- 



