Microtomes: their Choice and Use 4 247 



wheel has been made, the sector is returned to its position 

 by means of a strong spiral spring. To prevent the wheel 

 returning with the sector, there is an arrangement for 

 gripping the micrometer screw, which, though sufficient to 

 prevent any backward movement of the wheel by the action 

 of the spring, does not hinder the forward movement. 

 This gripping arrangement is controlled by the long 

 steel pin seen at the base of the instrument to the 

 right of the focusing apparatus ; and this pin also fits all 

 the other capstan-headed screws of the instrument. A 

 silver-plated tray is fitted to the base of the microtome. 

 All the working parts are of polished steel, the heavy 

 casting being japanned. The weight of the instrument 

 (as Fig. 65), with a tramway 25 centimetres long, is with 

 its mahogany case about 32 Ibs. The microtome may 

 have other additions to its equipment. The movement of 

 the knife-carrier in its tramway may be, if desired, effected 

 by mechanical means. This is not at all necessary in the 

 ordinary way, but useful for very fine work, and, as the 

 mechanism is removable, the instrument may be used with 

 or without its aid. The object -jaws may be further elabo- 

 rated by the use of the Naples object -holder. By its means 

 the object may be finally orientated by rack and pinion 

 about two axes ; this is useful in certain work, but not at 

 all necessary either to the beginner or the advanced student 

 excepting in special cases. Its great advantage lies in the 

 fact that the axes of movement lie very close to the object, 

 hence the object may be considerably orientated during the 

 process of cutting without greatly altering its elevation, 

 and the object may be raised at one end previous to cutting 

 each section, thus giving sections thicker at one end than 

 the other. The Naples object-holder is absolutely necessary 

 with the Eivet model microtomes, but not so with the 

 Schanze, as this already possesses a good orientating 

 arrangement, as has been described. 



Again, by means of another wheel to the micrometer 

 screw below the one already described, one is able by the 



