THE PARAFFIN iMETHOD 123 



should yield good ribbons at 2 and 3 yu, on a rotary microtome; and on 

 a good sliding microtome, should yield good sections at 1^, or even 

 thinner. The ideal temperature for 1-0.5 ^ sections, with 50° or 52° C. 

 paraffin, is — 2° C. In warm weather, the microtome should be cooled 

 on a block of ice and the knife and object must be kept cool by holding 

 a small piece of ice against them every two or three minutes. A small 

 piece of ice can be kept against the knife or holder by a rubber band. 

 The ice is only a necessary evil. Try to arrange your time so as to do 

 your cutting in cold weather. Let the room get cold, put on a warm 

 coat, and go to work. 



If you are still in the microtome knife stage, get two good 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 30 or 40 di- 

 ameters, 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, will cut good sections, but it requires both 

 time and skill to keep the edge perfect. Of course, for large sections, 

 more than 18 mm. in diameter, a regular microtome knife is necessary. 

 To get the right bevel, use a "back." For knives longer than 4 inches, 

 we prefer to have a back only on the upper side. For blades 4 inches or 

 less in length, the tube-like back, giving a bevel like that of a safety 

 razor blade, is satisfactory. 



With the Watts safety razor blade in the holders shown in Figures 

 3 and 4, we have cut smooth ribbons of Selaginella strohili, sections 

 through the sporangium region of the whole plant of Isoetes, sections 

 of stems of Cucurbita, in fact, we have not used an ordinary micro- 

 tome knife for cutting paraffin ribbons for more than 15 years. Many 

 fail at the first attempt and go back to the continual drudgery of 

 sharpening microtome knives. If the holder shown in Figure 4 is 

 placed in the rotary microtome at the angle used for a microtome 

 knife, failure is certain; for the blade, which is bent into a curve, will 

 scrape rather than cut. A study of Figure 5 should enable anyone to 

 secure the proper angle. 



