The Paraffin Method 111 



cooling. If paraffin cools slowly it crystallizes and does not cut well. 

 The layer of paraffin should be just thick enough to cover the objects, 

 not only as a matter of economy, but because a thick layer retards 

 the cooling. Very small objects, like the megaspores of Marsilea, 

 ovules of Silphium, etc., may simply be poured out upon a cool piece 

 of glass. In this way very thin cakes are made, which harden very 



rapidly. 



CUTTING 



As soon as the paraffin is thoroughly cooled, it is ready for cutting. 

 Trim the paraffin containing the object into a convenient shape, and 

 fasten it upon a block of wood. Blocks of pine f inch long and f inch 

 square are good for general purposes. Put paraffin on the end of the 

 block so as to form a firm cap about f inch thick. Warm the cap 

 and the bottom of the piece containing the object, and press them 

 lightly together; then touch the joint with a hot needle, put the 

 whole thing into cold water for a minute, and it is ready for cutting. 

 Cutting can be learned only by experience, but a few hints may not 

 come amiss: 



a) Keep the knife sharp. There should be two hones, one for 

 use when the knife is rather dull and the other for finishing. For 

 the first hone, nothing equals a fine carborundum hone. About 

 5.5X22.5 cm. is a good size. A hard Belgian hone, of the same 

 size, may be a little better for finishing. Flood the stone with water, 

 and rub it with the small slip which accompanies all high-grade hones; 

 this not only makes a lather which facilitates the sharpening, but 

 it also keeps the surface of the hone flat. As soon as the edge of the 

 knife appears smooth and even under a magnification of thirty or 

 forty diameters, the sharpening is completed with a good strop. It 

 is better to sharpen the knife every time you use it. A first-class 

 microtome knife, in perfect condition, is unsurpassed for cutting 

 paraffin sections, but it requires both time and skill to keep the edge 

 perfect. More than ten years ago we began to experiment with the 

 Gillette safety-razor blade and devised several holders for it, some 

 of them more or less successful. Mr. Strickler finally perfected a 

 holder which has already been mentioned. In using this holder 

 the blade should not project more than 1 mm. 



