374 MICROSCOPIC SECTIONING [Cn. XI 



some carborundum powder, i.e., 6o-minute carborundum, that which 

 is so fine that it remains in suspension in water for 60 minutes, or one 

 may use diamantine or jewelers' rouge. 



Stropping. With the back foremost draw the knife lengthwise 

 of the strop with a broad sweep. For the return stroke turn the 

 edge up as in honing. Continue the stropping until a hair can be 

 cut i to 2 centimeters from where it is held. (See also the hones and 

 strops and the methods of procedure recommended in the catalogues 

 of microscopical manufacturers.) 



607. Free-hand sectioning. To do this one grasps the section 

 knife in the right hand and the object in the left. Let the end to be 

 cut project up between the thumb and index finger. One can let 

 the knife rest on the thumb or index finger nail and with a drawing 

 cut make the section across the end of the piece of tissue. By prac- 

 tice one learns to make excellent sections this way. If the whole 

 section is not sufficiently thin, very often a part will be and one can 

 get the information needed. 



608. Sectioning with a hand or table microtome. The tissue is 

 held by the microtome and moved up by means of a screw. The 

 knife rests on the top of the microtome and is moved across the tissue 

 by the hand. Microtomes of this kind are excellent. No one need 

 wait for expensive automatic microtomes to do good sectioning. 

 With a good table microtome, the knife being guided by the hand 

 or hands of the operator, he can make straight cuts as for paraffin 

 sectioning, or drawing cuts as for collodion work. 



609. Sectioning with a freezing microtome. In this method of 

 sectioning the tissue is rendered firm by freezing and the sections are 

 cut rapidly by a planing motion as with paraffin. Now the most 

 usual freezing microtome is one in which the freezing is done with 

 escaping liquid carbon dioxid. The knife should be very rigid. A 

 carpenter's plane blade is often made use of. The tissue may be 

 either fresh or fixed. If alcohol has been used it must be soaked out 

 of the tissue by placing it in water. Sometimes tissues are infiltrated 

 a day or two in thick gum arabic mucilage before freezing. Drop a 

 little thick mucilage on the top of the freezer, put the tissue in the 

 mucilage, and turn on a small amount of carbon dioxid. It will 



