COLLECTION OP SPECIMENS 169 



taken up with the pair of curved forceps, and is 

 gently lowered onto the jelly, beginning from 

 the extreme left-hand side, and driving the jelly 

 (and too often, alas ! the specimen also) before it, 

 as it is allowed to fall gradually into place. This 

 is a matter of no little delicacy, for if great care is 

 not taken, and especially if sufficient jelly is not 

 used, a large bubble of air will often make its 

 way in at the last moment, and then there is 

 generally nothing for it but to raise the cover- 

 glass, and begin over again. It usually happens 

 that the object gets more or less displaced by the 

 putting on of the cover-glass, and requires some 

 adjustment before the jelly is allowed to set. One 

 of the bent dissecting-needles will now be of great 

 service, for a considerable amount of rearrange- 

 ment can be effected by its means ; stray air- 

 bubbles, too, may often be thus removed without 

 disturbing the cover-glass. Should the quantity 

 of liquid jelly used not be sufficient to fill the 

 whole space under the cover-glass, a small addi- 

 tional amount must be introduced by means of the 

 pipette, and it is also advisable not only to see 

 that the jelly comes right up to the edge of the 

 cover-glass, but to let some extend beyond, all 

 round the margin of the cell; this provides for 

 shrinkage in the gelatine, which is one of the 

 chief ingredients of the jelly. Two cells may very 

 conveniently be mounted on each slide; for in- 

 stance, one may contain a small portion of the 



