542 SUMMARY OF CURRENT RESEAROUES RELATING TO 



cylinder, aiul this projecting part is cut off by tlic knife. Tlio upper 

 edge of the cylinder is fixed into a metal i)lato, usually covered with 

 glass, on which the knife is guided by the hand, and a quiet and 

 steady movement of the same obtained. 



The possibility of making fine sections docs not depend solely on 

 the small and uniform raising of the preparation, but especially ou 

 the fixing of the preparation, and the impossibility of lateral displace- 

 ment from the liorizontal. The slightest imperceptible change of 

 direction of the object must produce unevcnncss of the section. 



In cylinder microtomes of older construction the preparation is 

 clamped or imbedded in a glass tube ; in others (Ranvier, Gudden, 

 Oschatz) tlie cylinder is closed below by a plate which is moved by a 

 screw. The hollow cylinder is filled with paraffin, spermaceti, &c., 

 and the preparation imbedded in the mass, so that fine rings of the 

 imbedding mass arc removed with the sections. Unless the imbed- 

 ding mass is quite close to the wall of the cylinder it is not firmly 

 fixed, and even sections cannot possibly be made. But this is impos- 

 sible if the imbedding mass is to be raised in the cylinder, and, 

 further, all imbedding masses contract on cooling. It is a very 

 difficult matter to make micrometer-screws faultless, i. e. with abso- 

 lutely reguhir distances of the threads. Even if it were possible to 

 make a faultless male and female screw, it would entail more trouble 

 and cost than a rail on which a slide is uniformly raised. 



Whilst, in preparations raised perpendicularly by a screw pre- 

 cision can hardly be obtained for 1/200 mm., slide microtomes aro 

 now made which raise the object 1/1000 mm. A further objection 

 to the screw micrometer is the wearing of the scrcw% whereas a slide 

 microtome, when properly used (i. e. when one does not always use 

 the slide only at one place, but allows it to traverse, wlien possible, 

 the whole rail), is always better for use, as the slide is always carried 

 symmetrically over the rail. 



Attempts have been made to remedy the defects of screw micro- 

 tomes of older (cylinder) construction. The movability of the object 

 in the cylinder and the difficulty of fixing it satisfactorily have been 

 abolished by a slide, which carries the preparation in place of the 

 cylinder. The preparaticm is tightly fastened to it by a clamp. This 

 method of fastening and raising offers more advantages than the 

 earlier ones, but one must not overlook the fact that the grooves and 

 edges of the slide must be made to fit as accurately as i^ossible, and 

 that even slight wearing produces a loosening of the slide and an 

 ajipreciable, if slight, movableness of the object. 



In the improved screw microtomes, as also in those which carry 

 the preparation on a slide which is moved in a vertical direction, the 

 screw should, after use, be turned to the end. By use a wearing of 

 the screw as well as of the slide-guide is inevitable, with consequent 

 loosening and inaccuracy of raising. 



In all microtomes which are used for fine work the knife is no 

 longer guided by the unaided hand, but by a slide which runs in a 

 horizontal plane, and to which the knife is screwed. The position 

 of the edge is of great importance in the preparation of fine sections. 



