ZOOLOGY AN!) BOTANY, MICROSCOPY, K'I'C. Jl\) 



(3) Cutting-, including- Imbedding- and Microtomes. 



Theory and Practice of Whetting.* — L. W. Ssobelew discusses this 

 important subject at considerable length, with especial reference to 

 microtome and surgical blades. The former is naturally of most interest 

 to microscopists, but both classes of blades involve the principles of 

 wedge and saw. Most knives have a wedge-shape and numerous tiny 

 teeth on the edge. A knife, when used, is not only pushed deep into the 

 tissue, but also acts as a saw. The best treatment of a tissue is attained 

 by making the wedge as thin and as sharp as possible ; the saw being as 

 slender, and its teeth as small, as possible. In whetting a knife the 

 inclination of the blade is of the first importance. The proper angle of 

 inclination may be attained freehand by an expert ; but a- safer method 

 is to fasten the blade in a little tube- shaped frame slit longitudinally for 

 the reception of the back of the blade. The blade in its frame is then 

 placed on the hone, the edge forwards, and is drawn towards the operator, 

 the movement commencing at the heel (i.e. the part nearest the handle) 

 and finishing at the knife-point. A blade with a curved edge will also 

 require a correspondingly curved stroke. The knife should then be 

 reversed and the other side similarly treated. Pressure is scarcely needed, 

 the weight of the knife should suffice. Pressure is apt to favour a " furred " 

 edge, which, however, is unimportant beyond the useful edge of the 

 instrument. The best way of testing an edge is by the thumb. For 

 this purpose the knife-handle is held in one hand and its back rests on 

 the four fingers of the secondhand, the thumb of which gently feels from 

 above the keenness of the edge. This co-ordination of the two hands 

 secures great delicacy of touch and a clear perception of the character of 

 the edge — whether it is sharp, or turned, or bowed— and, finally, whether a 

 coarse-grained or a fine-grained stone should be used. With practice, it 

 may even be judged whether the knife can cut sections 5 /* or 10 //. thick. 

 The critical sensation is obtained at the moment of lifting off the thumb, 

 and therefore only short distances, 2 or 3 cm. at a time, should be tested. 

 The author, in the application of this test, has never cut himself deeply 

 •enough to draw blood. For surgical knives such a mode of testing would 

 be unsuitable, as the existence of any slight wound might be a source of 

 danger. The author therefore recommends that such a knife held at a 

 sharp angle should be pressed on the hair at the back of the operator's 

 head. If it cuts the hair freely, it is ready for stropping. Among 

 materials suitable for hones he finds that : — 



1. Natural red Russian sandstone, moistened with water, soon gives 

 an edge, although somewhat of a coarse one. 



'1. Grey natural whetstones, moistened with water or oil, act some- 

 what slowly, but give a very fine edge. 



3. Yellow Belgian whetstones (said to be artificial, but not always 

 homogeneous), moistened with oil, give a fine edge very suitable for 

 microtome knives. 



4. White American natural whetstones, called Arkansas or Mississippi 

 stones, are slow, but give a very good edge ; especially suitable in cases 

 where stropping is impossible, e.g. with dentists' and oculists' instruments. 



A whetstone should always be moistened with a suitable fluid (e.g. 



* Zeitschr. wiss. Mikrosk., xxvi. (1909) pp. 65-79 (7 figs.). 



