THE MICROTOME AND SECTIONING. 



33 



struments, as the so-called rocking microtomes, are so specialized that they 

 only cut paraffin objects when the knife is transversely placed. Others 

 have a more general function, celloidin as well as paraffin objects being 

 sectioned with the knife in any position. To the latter class belong the 

 sliding microtomes. 



In figure 4 is shown an instrument which may be recommended 

 for general laboratory work. This instrument consists of a horizontal 

 base which rests on the table, and a vertical plate (a}, and a slide (/>) which 

 supports a block (Y), to which is fastened a knife by means of a thumb- 

 screw (//). On the other side of the vertical plate is a metal frame (<?), 

 into which are fastened the paraffin and celloidin blocks ; this frame is 

 attached to a slide (/"), which may be elevated or lowered by a feed (). 

 This feed consists of a micrometer screw acting on the lower surface 

 of the slide. The micrometer screw is provided with a milled head, 

 divided into a definite number of parts which bear a definite rela- 



Fig. 4. Laboratory microtome. 



tion to the pitch of the micrometer screw. The instrument shown 

 in the figure is further provided with a lever (//), which may be 

 so adjusted as to move the milled head on the micrometer screw 

 i or any given number of notches at each movement of the lever ; 

 and as each notch on the milled head has a value of 5 microns 

 (oinnr f an i ncn )> every time the milled head is moved i notch 

 (toward the manipulator) the slide carrying the clamp holding the tissue 

 is elevated 5 microns; 2 notches would elevate the tissue 10 microns 

 (loinr f an i ncn ) > 4 notches, 20 microns (y^Vg- f an i ncn )> etc. It 

 is not essential to have a lever attached to the instrument as above 

 described, although this is very convenient ; if not present, the milled 

 head is moved the desired number of notches with the hand. 



Minot has recently devised two kinds of microtomes which deserve spe- 



ial mention, and are especially to be recommended for accurate work. 



)ne of these (see Fig. 5) is known as the "Precision Microtome." It 



onsists of a square frame made of cast-iron, to which the knife is fastened. 



3 



