ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



543 



Fig. 120. Fig. 121. 



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The knife should pass through the object to be cut in as nearly 

 as possible its whole length. If we make a drawing so as to appre- 

 ciate the course of the knife better, we obtain (fig. 120) an ideal 

 knife-guidance by hand. In this ease the knife m runs in a sagittal 

 direction obliquely through the preparation Pr, and thus passes through 

 it in its entire length. (The dotted lines 

 show the course of the individual sections of 

 the knife.) If we alter this drawing only by 

 bringing the knife into the position shown 

 in fig. 121, and cut in the inverse direction 

 from left to right, in which we draw the knife 

 in a " sagittal " direction towards us, we have 

 the course of the knife on the microtome, 

 and the same figure as before, only that, in 

 the latter case, the knife must be a trifle 

 longer, and the position of the preparation to 

 it is more oblique. If we lay, in a special 

 case, particular value on the direction in 

 which the knife glides through the object, 

 we can easily alter the position of the object 

 in such a way that the individual segments 

 of the edge pass through the preparation in 

 precisely the same way as in fig. 120. The 

 dotted line Pr' marks the position thus 

 altered. 



The longer the knife is in proportion to 

 the preparation to be cut, the less must be 

 the pressure applied in cutting ; the shorter 

 the distance over which the knife is used, 



the greater must be the pressure which is applied with similar size of 

 the object, and the more will the edge crush and chisel. A surface 

 which has been cut through is more smooth and uniform than one 

 which has been pressed on, and the greatest possible use of the edge 

 of the knife offers the best guarantee for perfect sections. 



The position of the surface of the knife to the cut 'surface must 

 also be taken into account. A knife must be applied the more flat the 

 finer the piece to be cut ; if the knife is placed in a steep direction, 

 the edge scrapes. The finest and most perfect sections are obtained 

 with a knife when it is in such a position that the side of the knife 

 which is turned towards the cut surface only touches the object at the 

 extreme margin of the edge. 



Above all it is necessary to test the edge of the knife, and the 

 shape of the cross-section of the knife. All our ordinary table, bread, 

 or meat knives in their cross-section have the form of a wedge with 

 straight sides, i. e. of an isosceles triangle (fig. 122 a). If it is blunt 

 it should have a steel passed over it, applying it not perfectly flat, but 

 at the sharpest possible angle to it ; the result of this being that the 

 whole surface is not ground, but only its extreme edge, so that the wedge 

 assumes a pentagonal instead of a triangular form, as is shown on an 

 enlarged scale in fig. 122 h. The newly made surfaces do not converge 



