180 The Microscope. 



lever oscillates. At the cutting- end of the lever a spring piolls the 

 lever down and effects the sectioning and also the adjustment for 

 the next section. The lever is pushed over and adjusted for the 

 successive sections by a hollow screw, through which passes the 

 trunnion on the side away from the knife. This screw is fixed to 

 a toothed wheel, three inches in diameter, which revolves close by 

 the side of the oscillating lever. The toothed wheel and screw is 

 actuated by a pawl fixed to the side of the lever near the handle. 

 The number of teeth which this pawl can pass in a single vibration 

 downward is controlled by a fixed stop, screwed into the under side 

 of the oscillating lever near the handle : the end of this stop striking 

 on the top of the bed-plate thus brings the lever to rest at a constant 

 point in its downward excursion. An adjustable sector by the side 

 of the toothed wheel throws the pawl out of gear after a given 

 radius of the wheel has been turned through an arc embracing the 

 desired number of teeth. This adjustment is also effected before 

 the block, containing th e object to be cut, reaches the edge of the 

 knife. The adjustment for the next section is therefore effected 

 while the surface of the block is not in contact with the under side 

 of the knife, so that no flattening or scraping effect is produced on 

 the surface of the block in its upward passage past the knife. 



The movement of the vibrating lever being arrested at each 

 down stroke at one point, and the pawl which catches into the 

 notches in the toothed wheel being released at any desired point by 

 the action of the adjustabl e sector, it is possible to adjust the appar- 

 atus with great accuracy for cutting sections of any desired thick- 

 ness. If a given radius of the wheel is moved through the are 

 embraced by a single tooth, sections are cut having a thickness of 

 only Tooo u o^ ^^ i°°^' °^' -0025 mm., — a thickness which is only 

 practically possible with paraffine embedding and a very keen razor. 

 If more teeth are taken by the pawl, any thickness of section is pos- 

 sible up to about ^^o of an inch, or .0625 mm.* 



A freezing-attachment, which has lately been appended to the 

 apparatus, shows that frozen sections can be made with as great 

 rapidity and success as those cut from objects embedded in the 

 paraffine block, and very nearly, if not quite, as thin. The freezing 

 attachment is as simple and efficient as the self-adjusting and cut- 

 ting devices of the instrument. Other auxihary apparatus makes it 

 possible to cut celloidin sections. This is effected by means of alco- 

 hol conducted by a tube fi'om a reservoir to the knife, over which the 

 fluid will run and drain into a tray below in such a way as not to 



* The screw which adjusts the block for cutting has exactly fifty threads to the inch, 

 and there are two hundred teeth on the periphery of the toothed wheel. The value of a 

 single tooth is, therefore, 1-50x1-200=1-10000 inch. 



