RAZOR-GUARDS. 187 



The student having become' furnished with a microtome, 

 his next selection must be a knife or razor, and in his 

 choice he must remember that it is absolutely necessary 

 for the whole of the back and edge to lie in the same plane 

 both must slide together over the top plate with perfect 

 smoothness of motion. The knife or razor should be 

 hollow-ground, flat-ground knives and razors being re- 

 served for cutting wood or bone, where great strength is 

 required. The cutting instruments must be kept to a very 

 keen edge by use of a Washita oil-stone, combined with 

 the application of a good razor-strop. It requires practice 

 to put on the keenest edge. Perhaps a lesson from a 

 barber would on the whole be a cheaper and speedier way 

 of learning. 



Mr. M. H. Stiles has described an arrangement by which 

 the blade of the razor is kept from contact with the plate 

 of the machine, by two small screw-clamps he terms " razor- 

 guards " (Fig. 176). These slide on the blade, one being 

 fixed at each end, and kept in position 

 by a small screw in the upper side of 

 each. They are f ths of an inch wide 

 and are made from sheet brass ^th of FlG " I7<5 ' 



an inch thick. They ensure a smoother and more steady 

 motion than by cutting in the old way, and the edge of the 

 razor is preserved in much better condition as it touches 

 nothing but the substance to be cut. 



Let us now consider the cutting operation. If we wish 

 to cut a section of the stem of the horse-chestnut (sEscufas 

 hippocastanum), Fig. 98, say in a line with the axis of 

 growth, we shall find it rather difficult, without first 

 embedding in some agent to hold it firmly, and without 

 undue pressure upon any one part. Formerly a mixture 

 of equal parts of beeswax and olive oil was used for this 

 purpose, and Professor Rutherford advises a mixture of 



