288 Transactions of the Society. 



and the pressure between them is large ; this prevents dust from 

 getting in and ensures an extremely thin film of oil, which will 

 not vary in thickness by an appreciable amount. These bearings 

 are a slightly modified form of the knife edge and plane used with 

 such success in the Eocking Microtome. 



Another cause of irregularity in the thickness of the sections 

 is the looseness produced by the wear of the various slides. 

 Usually the slides require the attention of a skilled mechanic for 

 their proper adjustment. In some instruments the object is carried 

 on a reciprocating slide with a comparatively long travel ; this 

 slide will wear, and then precise cutting becomes impossible until 

 skilled attention has been given to the instrument. 



In the new microtome the geometric principle is adopted in 

 the chief moving parts, ensuring accurate working without shake, 

 even after much wear of the rubbing surfaces. The wear at these 

 surfaces is, however, small, but even if it were large it would be 

 all taken up automatically by springs, and thus no shake is possible. 

 Skilled attention is not required to keep the microtome in good 

 order. 



Manipulation. 



In cutting sections with a microtome it is advantageous that 

 the force acting on the object during cutting should be transmitted 

 accurately to the hand. This is markedly the case in this micro- 

 tome. The connexion between the hand and the object is rigid, 

 whereas in the Eocking Microtome the cutting force is due to a 

 spring. But what is more important, the friction and inertia of 

 the moving parts is small. In the majority of sliding microtomes 

 the friction is considerable, and in microtomes of the Minot pattern 

 there is a considerable amount of inertia of the moving parts, and 

 the friction is not small and is somewhat uncertain in amount. 

 The connexion between the hand and the object is by means of a 

 crank and connecting rod, and thus the ratio of the force on the 

 hand to the force on the object varies greatly during the rotation 

 of the handle. The ratio of the velocity of the hand to the velocity 

 of the object also varies during the rotation of the handle. A 

 variation of tlie ratios of the forces and velocities takes place in 

 the Universal Microtome, but the variation is much less. A fly- 

 wheel of considerable weight is usually fixed to the crank axle in 

 the Minot type of microtome. This increases the inertia of the 

 moving parts and ensures more uniform velocity of the hand and 

 less uniform velocity of the object relatively to the knife. We 

 find that if the cutting speed is too high, the edge of a hollow 

 ground knife vibrates and irregular work may be produced. If 

 the cutting speed is uniform throughout the stroke it is easier 

 to prevent it ever becoming too great, and if the inertia of the 



