OF MICROSCOPIC OBJECTS. 3 
be shown. He should bear in mind, too, that the same 
structure may well be submitted to various modes of in- 
quiry, and that possibly new modes may occur to him which, 
though they may not serve to prove anything directly, may 
yet become negative proofs. 
Ist. Such tests and agents as render transparent or 
translucent some tissues but not others adjacent, or make 
some more conspicuous than others without colouring them, 
or at least but faintly. 
2nd. Staining materials or fluids, which colour either all 
the tissues to be displayed, or some particular part or parts 
of them, thus making such tissues or parts more conspicuous 
when subsequently examined or preserved in a colourless 
medium. 
srd. Hardening agents or solutions, by the effect of 
‘which tissues naturally so soft as to break down or be other- 
wise unmanageable under manipulation, are made firm 
enough for section, or for such examination as may suffice to 
to discriminate (or “ differentiate”) their parts, without 
either disturbing or confusing their structural relations. 
Ath. Softening agents of animal and vegetable tissues. 
5th. Solvents of the same. 
6th. Solvents of calcareous matter. 
7th. Solvents of siliceous matter. 
8th. Solvents of oily and fatty matters. 
9th. Polarized light, by the agency of which structures 
and organs may often be optically differentiated as a pre- 
liminary to other modes of investigation. 
10th. Electricity and heat. 
11th. The moist chamber. 
In dealing with structures by means of the agents com- 
prised under the first division of our list, a very frequent 
_ and necessary preliminary is the teasing out or separation 
of fibres by means of two sharp needles set in convenient 
handles. But it must be remembered that an appearance 
of structure, where it does not really exist, may easily be 
thus produced. It is often necessary, also, that the object 
BQ 
