218 THE AMERICAN MONTHLY [July, 



the block containing the object to be cut reaches the edge of the 

 knife. The adjustment lor 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 pro- 

 duced 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 adjustable sector, it is possible to ad- 

 just the apparatus with great accuiacy for cutting sections of 

 any desired thickness. If a given radius of the wheel is moved 

 through the arc embraced by a single tooth, sections are cut 

 having a thickness of only ^i^JjjQ of an inch, or .0025 mm. — a 



thickness which is only practically possible with parafBne em- 

 bedding and a very keen razor. If more teeth are taken by the 

 pawl, any thickness of section is possible up to about ^ of an 

 inch, or .0625 ram.^ 



A freezing attachment which has lately been appeuded 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 ad- 

 justing and cutting devices of the instrument. Other auxiliary 

 apparatus makes it possible to cut celloidin sections. This is 

 effected by means of alcohol conducted by a tube from a reser- 

 voir to the knife, over which the fluid will run and drain into 

 a tray below in such a way as not to come in contact with any 

 other parts of the machine. This tray fits into a recess in the 

 side of the bed-plate of the instrument just below the knife, and 

 into this tray the celloidin sections may be allowed to drop as 

 fast as cut. 



The paraffine-holders are square and seven-eighths of an inch 

 in diameter, so that a block of that size may very readily be 

 sectioned. For the botanist, one of these holders is provided 



1 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, ^^ x 2J0 = 10000 ^"^h. 



