140 The Preparation of Microscope Slides 



The block may be left in water for any reasonable length of time, but 

 if it is to be stored for days or weeks it is much better kept in a 5 per cent 

 solution of glycerin in 70 per cent alcohol. There seems to be a widespread 

 delusion that, because an object must be perfectly dehydrated before be- 

 ing impregnated with wax, it must subsequently be kept out of contact 

 with fluids. Nothing could be further from the truth. As will be discussed 

 later, when dealing with the actual technique of sectioning, it is often 

 desirable to expose a portion of the object to be sectioned and leave it 

 under the surface of water for some days in order to get rid of the brittle- 

 ness that has been imparted through the embedding process. Blocks that 

 have been stored dry for a long period of time should always be soaked in 

 a glycerin-alcohol mixture for at least a day before sectioning. This is 

 particularly important with plant material. 



It is, in any case, undesirable to section a block as soon as it has been 

 made, because it is necessary for successful sectioning that the block be 

 the same temperature throughout. If a block is made in the evening, it is 

 better to take it out of the water and to leave it lying on the bench over- 

 night, so that the temperature may be stabilized. 



Choice of a Microtome. Microtomes may be broadly divided into two 

 classes. In the first of these the block remains stationary, while the knife 

 is moved past it. In the second group are those in which the block moves 

 past a stationary knife. The first class ( Fig. 104 ) is commonly known as a 

 Schantz microtome ( after the original German model ) . They are made by 

 several manufacturers, and are rarely used for the preparation of serial 

 sections. They have the advantage that relatively large blocks may be cut 

 but they have the disadvantage that no ribbon can be obtained broader 

 than the width of the knife. 



Fig. 104. Sliding microtome. 



