Chapter V: The Paraffin Method 47 



a) The commonest cause of rolling is the hardness of the paraffin. 

 This may sometimes be remedied by one or more of the following means': 

 (1) warming the knife with the breath; (2) cutting in a warmer room; 

 (3) placing a lamp or burner near the imbedded object; (4) warming the 

 knife very carefully by holding the back on a warm paraffin bath; 

 (5) cutting the sections thinner; (6) reimbedding the object in softer par- 

 affin. 



6) The tilt of the knife may be too great (step 12). 

 c) The knife may be dull. 



5. The Sections Split Longitudinally or Are Crossed by Parallel Scratches. 

 a) Look for a nick in the edge of the knife. Cut in a new place on the 

 knife or sharpen it. 



b) A bit of grit may have gotten into the object or the paraffin. 

 Reimbed after carefully cleaning the object in the clearing fluid. 



c) Tissues may contain hard substances (lime salts, silica, crystals 

 precipitated from fixing reagents) which have been imperfectly washed 

 out. It is best to take an entirely new piece of tissue in which these 

 defects do not exist. 



d) The tilt of the knife may be too great (step 12). 



e) The object may be too large to cut in paraffin. Try smaller pieces 

 of tissue or use the celloidin method. 



6. The Knife Scrapes or Rings as It Passes Back over the object after 

 having cut a section. 



a) This is sometimes caused by a knife with either too great or too 

 little tilt (step 12). 



6) The object may be too tough or hard to cut in paraffin without 

 springing the edge of the knife ( see 76). 



7. The Sections Vary in Thickness; the machine cuts one thick and one 

 thin or misses a section. 



a) This may be caused by the imperfect mechanical construction of 

 the machine. Old machines in which the parts are worn are especially 

 liable to this defect. It may be remedied to some extent by tightening 

 up the parts of the machine. 



6) The object may be too hard for the knife to cut and, as a conse- 

 quence, the edge of the knife springs. When tough or hard objects must 

 be cut, use an old microtome knife or a sectioning razor. See if there is 

 not some means of softening such a tissue without obscuring the micro- 

 scopical structures sought. 



c) Either too great or too little tilt may cause the defect (step 12). 



d) See that the disk bearing the object is securely clamped in the 

 machine. 



8. The Object Crumbles or Drops out of the Paraffin as Cut. It has 

 probably been insufficiently penetrated by paraffin. Some of the 



