ECHINODERMATA. 147 



that no water may be retained in the interstices of the network, 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 of 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 be observable, or some minute air bubbles should 

 show themselves between the glass and the under surface, it is desirable 

 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 be in the balsam covering the part of the glass 

 on which it is laid. The surface now brought uppermost is then to be very 

 carefully ground down; special care being taken to keep its thickness uni- 

 form through every part (which may even be better judged-of by the touch 

 than by the eye), and 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 

 continual recourse to the Microscope during the latter stages of his work; 

 and he should bear constantly in mind, that, as the specimen will become 

 much more transparent when mounted in balsam and covered with glass, 

 than it is when the ground surface is exposed, he need not carry his 

 reducing process so far as to produce at once the entire transparence he 

 aims at, the attempt to accomplish which would involve the risk of the 

 destruction of the specimen. In ' mounting' the specimen, liquid bal- 

 sam should be employed, and only a very gentle heat (not sufficient to 

 produce air-bubbles, or to loossen 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 has been newly laid-on. 



539. 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 com- 

 pare 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 blowing of the cal- 

 careous dust from their surfaces, 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 grind- 

 ing of such sections), and then upon a large Water-of-Ayr stone whose 

 surface is ' true. ' When this level has been attained, the ground surface 

 is to be well washed and dried, and some balsam previously hardened is 



