738 HISTOLOGIG TECHNIC 



away from the operator, and the knife pushed from heel to toe, 

 the whole length of the stone. The motion being repeated, a sharp 

 edge is gradually acquired, which can be finished by the use of the 

 strop. 



In the use of the strop the motions are the reverse of those with 

 the hone, the back of the knife in this case preceding its edge as it is 

 drawn along the leather, and the draw should be from the toe to heel 

 of the knife. The angle, however, between the knife and the hone and 

 the knife and the strop should always be a constant one, and should 

 be such that the microscopical 'teeth' which are thus formed on the edge 

 of the knife should be directed obliquely toward its heel. 



In sectioning, the knife should be so placed in the microtome that 

 its edge crosses the paraffin-embedded object at right angles, and for 

 ribbon sectioning the paraffin block should be so trimmed that it forms 

 a perfect rectangle. In sectioning celloidin-embedded objects, the knife 

 should cross the object at as acute an angle as possible. With paraffin, 

 also, the stroke should be sharp and quick; with celloidin, somewhat 

 slower and rhythmic. The knife should remain dry when used with 

 paraffin; with celloidin both the knife and the object should be at all 

 times well moistened with 70 per cent, alcohol. 



STAINING 



The sections, having been cut, are at once ready for staining, pro- 

 vided they were embedded in celloidin. If paraffin was used for em- 

 bedding, the sections have first to be fastened to the slide. This is 

 accomplished in the following manner. 



The paraffin sections are properly arranged upon the surface of a 

 clean slide, a few drops of water from a pipette are allowed to flow 

 between the slide and the sections, so that the -latter float upon the 

 surface of the water; and the slide is gently heated over a small flame. 

 Thus the paraffin sections are straightened; care should be used not 

 to melt them. The excess of water is now carefully drained off and the 

 slide placed in an oven and heated to about 40 C. for several hours, 

 until thoroughly dried. Most tissues will now adhere firmly to the 

 slide. If, however, the tissue was fixed with solutions containing bi- 

 chromate of potassium the sections are liable to come off the slide, a 

 misfortune which may be avoided by the use of a celloidin adhesive, 

 with or without the previous use of Mayer's albumin. As a precautionary 



