Production of Sections 271 



may be used. The great desideratum is to cool the paraffin 

 to solidity at the greatest speed. 



Paraffin cooled slowly forms crystals of large and loose 

 texture with numerous air cavities; such conditions are 

 fatal to good section cutting. But when cooled rapidly, 

 the crystals are small, firm, and without air cavities ; the 

 mass is homogeneous, and is in the condition sought by the 

 microtomist. Figs. 73, 74, 75, which are photo-micro- 

 graphs by polarized light of sections of paraffin, make 

 further remarks unnecessary. 



If the objects are light-coloured and not easily seen in 

 the embedding basin when the wax is cold and semi-opaque, 

 some method of marking their exact position must be de- 

 vised. When cold, the embedded objects are extracted from 

 the basin in the following manner : After noting the position 

 of each object, carefully cut away the surplus wax around 

 each, leaving each object within a good-sized cube of 

 paraffin. Slight lateral pressure will then detach the cube 

 from the bottom of the basin ; if not, a little heat applied to 

 the bottom, combined with the pressure, will accomplish it. 

 The blocks of paraffin in which the objects are now embedded 

 must be reduced in size so as to leave the object, not in the 

 centre, but a little to one side of the block, the opposite 

 faces of which must be cut perfectly parallel to one another. 

 Bear in mind which is the upper side of your object, and 

 note that air-holes, or opaque places in the cube, will 

 necessitate re-embedding ; with care, however, this will very 

 seldom be required. The cubes of paraffin cannot be fixed 

 immediately in the jaws of the microtome, but are cemented 

 to the paraffin table usually supplied, and which fits into the 

 same socket as the jaws. It will, however, be found more 

 convenient to reject the usual paraffin table, and to cement 

 the block to small cubes of hard wood (not cork), and then to 

 place this in the jaws of the microtome. Fig. 76 illustrates 

 the method. The wooden cubes may be purchased 1 cubic 

 inch in size or larger from dealers in physical apparatus 

 at a few pence per dozen. It is well to roughen the surface 



