16 PRACTICAL MICROSCOPY. 



M, to which the tissue N is affixed. An ether-freezing attachment 

 may be substituted for the plate I. 



The milled-head is divided into one hundred parts, so that each 

 fraction of a turn raises the tissue -g- oW f an inch. 



The knife should possess an edge of the most exquisite keenness ; 

 and this holder admits the employment of almost any cutting instru- 

 ment. In order to the production of the best results, the knife 

 should be set at the most acute angle compatible with the use of the 

 entire length of the cutting edge, from heel to point. Both knife 

 and tissue are to be flooded with alcohol, in ordinary work, as in free- 

 hand cutting. A drip pan is provided, and is placed below the tissue- 

 carrier. A groove in the front upper edge of the base prevents the 

 pirit from flowing over the track, which., mixing with the lubricating 

 oil covering the latter, would interfere with the delicacy and ease of 

 the sliding motion. 



The value of this instrument is largely consequent upon its great 

 solidity the base weighing from eighteen to twenty-five pounds, with 

 the knife carriage correspondingly heavy. Just why such weight 

 and solidity are necessar} 7 , and contribute so largely to our success in 

 cutting sections, is not at once apparent. The microtome is now 

 made by Mr. L. Schrauer, of New York, in two sizes ; the smaller 

 carrying a knife fourteen, and the larger, eighteen inches. A smaller 

 pattern would present no special advantages over microtomes already 

 in use. 



SHARPENING KNIVES. 



In the majority of instances of failure to produce suitable sections 

 for microscopical work, the cause can be set down to dull knives; and 

 I would urge the student to practice honing, until able to put cutting 

 instruments in good condition. If he will but start properly, success 

 is sure. Nine-tenths of the microtomes are purchased because of 

 failure in free-hand work with a dull knife; but no advantage will be 

 gained by a machine, if 1 he student be incapable of keeping the knife 

 up to a proper degree of keen ness. 



A knife is a wedge, and for our purposes the edge must be of more 

 than microscopical tenuity it being impossible, with the microscope, 

 to discover notches and nicks if properly sharpened. 



It is impossible to secure the best results with indifferent tools. 

 The knife should be hard enough to support an edge, but not so hard 

 as to be brittle. The proper temper is about that given a good razor. 



We need at least two hones one comparatively coarse, for 

 removing slight nicks; and another, for finishing. The first part of 

 the work is best done by means of a sort of artificial hone made with 



