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. 
