ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



77 



the sliding part S P (fig. 10), into which the object-clamp tits, move 

 freely backward and forward iu B, its polished inclined surface being 

 firmly held by a spring against the point P which, in turn, is firmly sup- 

 supported on the slideway forming a part of the feed-mechanism, which 

 again, in turn, is independent of the up-and-down movement. This is 

 contrary to most microtomes, as in the majority of them the feed- 

 mechanism is dependent on the up-and-down movement, with the result 

 that sooner or later inequality of section-cutting results. In the Spencer 



FYG. 1 



Pig. 10. 



machine the feed-mechanism is composed of a rigid bearing on which the 

 feed-block F B, of which the point P is a part, is moved by the feed- 

 screw F S. As this block is moved towards the side on which the balance 

 wheel W is located, the sliding part S P is forced forward towards the 

 knife one half as much, because the polished surface resting against the 

 point P is set at the proper angle to accomplished this purpose. Thus 

 any imperfection in the screw is reduced by one half. As the screw is 

 cut with two threads to the millimetre and as it is revolved by a ratchet 

 feed-wheel with 250 teeth, each tooth represents a feeding of the object 

 forward one micron. The feed is so arranged that it can he set for 



