12 THE ELEMENTS OF VEGETABLE HISTOLOGY 



Microtome Technic. — Fix the block in the object- 

 clamp, placing thin strips of cork on the sides in con- 

 tact with the gripping surfaces. These cork strips 

 serve as buffers and allow one to draw up well on the 

 tightening screws with little danger of injury to the 

 block. After the block is firmly fastened, place the 

 knife in position so that it clears the block and object- 

 clamp. The position of the knife varies with different 

 embedding media. For objects embedded in paraffin 

 the knife should be placed at an angle to the block. 

 For objects in celloidin or collodion the knife should 

 be placed straight, so that the block strikes it squarely. 

 The knife should be adjusted so that the cutting 

 edge is slightly lower than the flat side, otherwise one 

 will have a continuous resistance to the block at each 

 stroke. After the block is secured and the knife prop- 

 erly placed, ascertain the thickness of section desired 

 and set the indicator at the proper figure. Adjust 

 the object-clamp so that the block is within a few 

 millimeters of the knife edge and trim away the par- 

 affin from the upper surface of the block until the 

 specimen is visible. Move the object-carrier toward 

 the knife with a slow and steady stroke, or if operating 

 a rotary machine, turn the wheel steadily and not 

 too rapidly. After cutting a few sections it is well 

 to examine them under the microscope. If the sec- 

 tions are not cutting true or through the desired plane, 

 use the adjusting screws located near the object-clamp 

 and make repeated adjustments until satisfactory 

 sections are secured. If the object is fairly straight 

 the adjustment need not be changed after true sections 

 are obtained. With bent or twisted objects con- 

 tinuous change of adjustment is necessary. Be sure 



