MICROTOMES. 391 



The microtome is an instrument of precision, which implies that it must be 

 treated with extreme delicacy and kept most scrupulously clean. It will be 

 found usually, when complaint is made against the microtome, that the complaint 

 is misdirected, and ought to be not against the machine, but against the owner. 

 The modern microtome necessarily has several adjustments, every one of which 

 must be exact and secure. If any one of them is imperfect or insecure, if any 

 of the movable parts is allowed to become corroded, or gummed up with oil, or 

 loose, or clogged with dust or dirt of any kind, the microtome will not and cannot 

 work as an instrument of precision. 



The knife used for cutting ought to be regarded as an integral part of the 

 microtome and as its most delicate and easily injured part. A perfect knife-edge 

 is the greatest treasure of the microtomist. To sharpen the knife satisfactorily 

 for fine section cutting is a really serious difficulty. A skillful person, however, 

 may get a good edge by using the very finest grade of oil-stone. No oil should 

 be used, but instead a mixture of equal parts of glycerin and water. Before the 

 knife is honed it must be made as clean as possible. The oil-stone itself also 

 must be cleaned with equal care, and the mixture of glycerin and water should, 

 if necessary, be filtered before using to keep it free from dirt. A single particle 

 of dirt may be the cause of making many microscopic notches in the edge of 

 a knife. A knife is well sharpened when its edge appears smooth and straight 

 under a magnifying power of twenty-five diameters. The microtome knife should 

 be as unlike a razor as possible. It must have a very thick back and be as 

 heavy and rigid as practicable, so that the actual cutting-edge may be as steady 

 and inflexible as it can be made. Knives of suitably heavy construction are now 

 furnished with all the best microtomes. 



