PREPARING SPINES 893 



.stone. It should then, after careful washing, be dried, first on white 

 blotting-paper, afterwards by exposure for some time to a gentle 

 heat, so that no water may be retained in the interstices of the net- 

 work which would oppose the complete penetration of the Canada 

 balsam. Next, it is to be attached to a glass slip by balsam hardened 

 in the usual manner- ; but particular care should be taken, first, that 

 the balsam be brought to exactly the right degree of hardness, and 

 second, that there be enough not merely to attach the specimen to 

 the glass, but also to saturate its substance throughout. The right 

 degree of hardness is that at which the balsam can be with difficulty 

 indented by the thumb-nail ; if it be made harder than this, it is 

 apt to chip off the glass in grinding, so that the specimen also breaks 

 away; and if it be softer, it holds the abraded particles, so that 

 the openings of the network become clogged with them. If. when 

 rubbed down nearly to the required thinness, the section appears to 

 be uniform and satisfactory throughout, the reduction may be com- 

 pleted without displacing it ; but if (as often happens) some inequality 

 in thickness should lie observable, or some minute air-bubbles should 

 show themselves between the glass and the under surface, it is de- 

 sirable to loosen the specimen by the application of just enough heat 

 to melt the balsam (special care being taken to avoid the production 

 of fresh air-bubbles) and to turn it over so as to attach the side 

 last polished to the glass, taking care to remove or- to break with 

 the needle point any air-bubbles that there may lie in the balsam 

 covering the part of the glass on which it is laid. The surface now 

 brought upper-most is then to be very carefully ground down, 

 special care being taken to keep its thickness uniform through every 

 part (which may be even better judged of by the touch than by the 

 eye), arid to carry the reducing process far enough, without carrying- 

 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 

 tiriual recourse to the microscope during the latter stages of his 

 work ; and he should bear constantly in nrind that, as the specimen 

 will become much more translucent when mounted in balsam and 

 covered with glass than it is when the ground surface i> exposed, he 

 need not carry his reducing process so far- as to produce at once the 

 entire trarrslucence he aims at, the attempt to accomplish which 

 would involve the risk of the destruction 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 cover and to reduce it further-, care being- 

 taken to harden to the proper degree the balsam which lias been 

 newly laid on. 



If a number of sections are to be prepared at once (which it is 

 often useful to do for the sake of economy of time, or- in order to 

 compare sections taken from different parts of the same spine), this 

 may be most readily accomplished by laying them down, when cut 

 off by the saw, without any preliminary preparation save the blow- 

 ing of the calcareous dust from their surfaces, upon a thick slip of 



