ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 545 



in which the shape of the knife is of the well-known form, whereas on 

 a curved leather strop (fig. 126) the surfaces which form the edge are 

 not plane, but convex, and meet in two arcs. 



The two figures differ especially in the angle at which the sur- 

 faces of the edge meet, and this angle is more blunt with soft strops. 

 It is hardly necessary to mention that the flat-lying knife should not 

 be pressed firmly during sharpening, as by pressure the leather is 

 pressed down and a convexity produced on the edge. 



As regards the direction of the under surface of the knife to the 

 cut surface, the knife-slide of several microtomes is provided with an 

 arrangement by which the inclination can be increased or diminished 

 at pleasure. If one is cutting a hard object, e. g. ebony or bone, the 

 knife must be set more steeply than for soft wood. But such an 

 arrangement seems superfluous, as one can by manipulation so adjust 

 the knife for hard substances that when screwed into the slide it has 

 a more slanting direction. A slight difference in the inclination of 

 the knife, as a rule, has little or no effect on the making of fine 

 sections. A sharp-angled knife always does its duty, while a blunt 

 one must be set more steej)ly, but scrapes rather than cuts. 



Uniform hardening and imbedding of the preparation is of primary 

 importance. Every paraffin imbeddiug-mass is only able to be cut with- 

 in certain definite limits of temperature, and the recent modifications, 

 which consist of admixture with tallow, spermaceti, oil, and other fats, 

 do not solve the secret of an equally suitable mixture for all tempera- 

 tures. Experience tells us that a mixture with a low melting point is 

 cut at a temjierature of 17° C. rather than at 25° C, and that one 

 generally gives up making sections in series at a temperature of 30° C. 

 and upwards, as the cooling in the water or spirit takes place too 

 quickly. 



In recent times too little attention has been paid to the fixing of 

 the knife. In the microtome of Fritsch the author first learned a 

 method by which the free end of the knife can be fixed. A spring 

 screwed into the knife-slide exercises, by means of a screw at its free 

 end, a slight pressure on the end of the knife, and keeps it perfectly 

 firm. 



When the earlier slide microtomes (e. g. Long's) are much used 

 their precision fiiils, and uniformly thin sections cannot be made. 

 Further, in cutting, the knife-slide runs stiffly, so that it has to be 

 constantly taken out and oiled. Thoma constructed a rail in 

 which the slide runs on 5 narrow (2-3 mm.) points, which give 

 great steadiness and easy motion to the slide when this is accurately 

 fitted. In Jung's microtome the knife-slide runs on five, the object- 

 slide on six points. Here too the whole rail should, when possible, 

 be traversed by the slide, as, by unequal usage of the long rail, the 

 knife glides inaccurately. 



A preparation slider, which moves the preparation forwards by a 

 screw instead of the hand, is found in Spengel and Jung's microtomes. 

 In the former the screw is of the same length as the instrument, and 

 is attached to the side of the instrument. In the latter the screw is 

 shorter, and possesses a pawl, by which the desired revolution of the 



Ser. 2.— Vol. V. 2 N 



