484 



OF ECHINOBERMATji. 



carry the reducing process far enough, without cariying it too 

 far. Until practice shall have enabled the operator to judge of 

 this by passing his finger over the specimen, he must have con- 

 tinual recourse to the microscope during the later stages of his 

 work ; and he should bear constantly in mind, that, as the speci- 

 men will become much more transparent when mounted in 

 balsam and covered with glass, than it is when the ground sur- 

 face is exposed, he need not carry his reducing process so far as 

 to produce at once the entire transparency he aims at, the 

 attempt to accomplish which would involve the risk of the de- 

 struction of the specimen. In "mounting" the specimen, liquid 

 balsam should be employed, and only a very gentle heat (not 

 sufficient to produce air-bubbles, or to loosen the specimen from 

 the glass) should be applied ; and if, after it has been mounted, 

 the section should be found too thick, it will be easy to remove 

 the glass covei-, and to reduce it further, care being taken to 

 harden the balsam which has been newly laid on, to the proper 

 degree. 



318. If a number of sections are to be prepared at once (and 

 it is often useful to do this for the sake of economy of time, or 

 in order to compare sections taken from diflierent parts of the 

 same spine), this may be most readily accomplished by laying 

 them down, when cut oiF by the saw, without any preliminary 

 preparation save the blowing the calcareous dust from their sur- 

 faces, upon a thick slip of glass well covered with hardened 

 balsam ; a large proportion of its surface may thus be occupied 

 by the sections attached to it, the chief precaution required being 

 that all the sections come into equally close contact with it. 

 Their surfaces may then be brought to an exact level, by rubbing 

 them down, first upon a flat piece of grit (which is very suitable 

 for the rough grinding of such sections), and then upon a large 

 Water-of-Ayr stone whose surface is "true." Wlien this level 

 has been attained, the ground surface is to be well waslied and 

 dried, and some balsam previously hardened is to be spread over 

 it, so as to be sucked in by the sections, a moderate heat being 

 at the same time applied to the glass slide; and this being in- 

 creased to a sufficient degree to loosen the sections without over- 

 heating the balsam, the sections are to be turned over one by 

 one, so that the ground surfaces are now to be attached to the 

 glass slip, special care being taken to press them all into close 

 contact with it. They are then to be very carefully rubbed down, 

 until they are nearly reduced to the required thinness ; and if, on 

 examining them from time to time, their thinness should be 

 found to be uniform throughout, the reduction of the entire set 

 may be completed at once ; and when it has been carried suffi- 

 ciently far, the sections, loosened by warmth, are to be taken up 

 upon a camel-hair brush dipped in turpentine, and transferred to 

 separate slips of glass whereon some liquid balsam has been pre- 

 viously laid, in which they are to be mounted in the usual man- 



