46 ELEMENTS OF APPLIED MICROSCOPY. 



The clearing agents just described depend for their 

 action solely upon the fact that their high index of re- 

 fraction prevents loss of the light passing through the 

 object. In the preparation of certain very opaque botan- 

 ical specimens, clearing solutions are used which act 

 chemically upon the tissues and actually dissolve- cer- 

 tain constituents which interfere with transparency. Of 

 these, strong aqueous solutions of potassium hydrate 

 or chloral hydrate are commonly used, and they must 

 be allowed to act upon the specimen for several hours or 

 even days. 



8. Section-cutting. — Since only very small objects can 

 be examined entire, it is necessary in many cases to pre- 

 pare thin sections of objects for examination with the 

 microscope. Even with fibres and similar objects which 

 can be easily studied in one dimension, cross-sections are 

 often desirable in order to gain an idea of their whole 

 structure. 



Sections of rigid objects may be cut directly with a 

 sharp razor; but few specimens are sufficiently hard to 

 be treated in this way. In general, plant and animal 

 tissues are so soft that they would give way even before 

 a sharp knife, and must be supported by imbedding 

 them in some material of firmer texture. If the sections 

 to be cut are not very thin, it is only necessary to sur- 

 round the specimen as a whole by a substance like pith; 

 if more delicate work is to be done, the tissue must be 

 permeated by some material like paraffin or celloidin 

 which will support each individual cell wall and make 

 it rigid. 



