and Laboratory Methods. 



2657 



e. g., in a razor edge for hairs of the beard, perhaps on exceedingly minute scale 

 even in a microtome knife for alternations of cell-walls in tissues, or, on a larger 

 scale, in a serrated bread-knife for a series of yielding gas cavities. 



II. HORIZONTAL SECTION, 



/. e., from top view of the edge, as when one looks down upon it. 



Ho/ie A. Surface of the edge (Fig. 3) appears as a nearly smooth, straight 

 and quite uniformly narrow band, with an average thickness of edge of 4 microns 

 (f^W inch), varying from 2 to 7 microns. 



None W. Surface of the edge (Photomicrograph, x 280, Fig. 4) shows a 

 little wider band, often finely scratched, with a central line (that of the low wedge 



Fig. 7. 



already referred to). It is slightly wavy from side to side in places, thickness 

 of the edge averaging 6 microns (40V0 '"ch), varying from o to 9 microns. 



Hone I. Surface of the edge roughly rounded to flat and well scratched, also 

 often wavy, with some sudden indentations from one side or the other. Its 

 breadth very variable, implying a thickness of the edge three or four times as 

 great as with the other hones, averaging 15 microns dyVo inch), varying from 

 4 to 23 microns. This edge surface is not shown in the photomicrographs, 

 (x 280, Figs. 5 and 6), as they were made exactly on vertical section. 



A little consideration will show, particularly as to the effects of Hones W 

 and I, that the true or effective width of the wedge of penetration, in use of 

 such a tool, is measured by a maximum thickness, that of the entire space that 



