44 Animal Microloyy 



coat the exposed surface of the object immediately before cutting each 

 section. If the collodion leaves a shiny surface or produces a mem- 

 brane when applied to the paraffin, it is not thin enough and must be 

 further diluted with ether-alcohol. Apply the collodion with a brush 

 with all excess of the fluid wiped away so that the brush is just moist. 

 The fluid should touch only the face of the block in which the object is 

 exposed. After applying, wait a few seconds for the solution to dry 

 before cutting. See also memorandum 9. 



9. Johnson's Paraffin-Asphalt-Rubber Method for brittle objects is a very 

 useful one. One part of crude India rubber cut into very small pieces 

 is mixed with ninety-nine parts of hard paraffin which has previously 

 been melted and tinged to a light amber color with a small amount of 

 asphalt ("mineral rubber"). The mixture is then subjected to a tem- 

 perature of 100 C. (not higher) for 24 to 48 hours, or left in a paraffin 

 oven at 60 C. for several days. Use only the supernatant fluid. It is 

 allowed to cool and remain cold until needed, because the rubber separates 

 out after a time if the mixture continues melted. Johnson (Journal of 

 Applied Microscopy, Vol. VI, p. 2662) recommends it as even better than 

 paraffin for all kinds of work for which paraffin is commonly employed. 

 Proceed as in the ordinary method, using xylol (not cedar oil) for dealco- 

 holization and also for clearing sections. 



10. Keep All Parts of the Microtome clean and well oiled with watch 

 oil or pure paraffin oil of 25 degrees. The instrument should be covered 

 when not in use. 



11. Keep the Microtome Knife Sharp. It should receive frequent strop- 

 pings. For sharpening the knife two hones are commonly used. 



Honing. If the knife is very dull it is first honed on a Belgian yellow 

 hone, an open-grained stone which cuts the metal of the knife rapidly. 

 The surface of the stone is kept moist with filtered kerosene oil or lathered 

 with palm-oil soap. After the nicks and other inequalities of the edge of 

 the knife have been removed, the honing is best finished on a good fine- 

 grained blue-water, stone. 



In honing the stone is laid flat on the table with its end toward the 

 operator and its surface properly lubricated. A very dull knife is ground 

 at first on the concave side only until it developes a fine "wire edge'' 

 along the full length of the blade. It is then ground on each side alter- 

 nately until the wire edge has disappeared completely. In grinding, the 

 knife must remain flat on the hone and pass lightly over the full length 

 of the surface, edge foremost in a diagonal direction from point to heel, 

 although itself remaining at right angles to the long axis of the hone. 

 The honing has been sufficient when all nicks and wire edges have disap- 

 peared and the knife, instead of catching and hanging when the edge is 



