OF MICROSCOPIC OBJECTS. 3 



ibe 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. 



1st. 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. 



3rd. 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 

 nther disturbing or confusing their structural relations. 



4th. 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. 



llth. The moist chamber. 



In dealing Tvith 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 



