COLLODIONISATTON OF SECTIONS. 149 



a good paraffin of 45 C. melting-point in a room in which the 

 thermometer stands at 16 to 17 C. (The temperatures 

 quoted apply to the case of rooms heated by an open fire, and 

 probably would not apply to the case of rooms heated by 

 closed stoves, such as are usual in Germany.) At 15 C. the 

 paraffin will be found a trifle hard. At 22 C. the proper 

 melting-point of the paraffin will probably be found at about 

 48 C., but my observations at these temperatures are less 

 extended. Secondly, the knife should be set square. Thirdly, 

 the block of paraffin should be pared down very close to the 

 object, and should be cut so as to present a straight edge 

 parallel to the knife edge ; and the opposite edge should also 

 be parallel to this. Fourthly, the sections ought to be cut 

 rapidly, with the swiftest strokes than can be produced. It 

 is evident that this condition can only be conveniently realised 

 by means of a sliding microtome; but it is by no means 

 necessary to have recourse to special mechanical contrivances, 

 as in CaldwelPs automatic microtome. The Thoma microtome 

 well flooded with oil is sufficient. But the automatic micro- 

 tomes are certainly most advantageous for this purpose, and 

 amongst them the Cambridge Rocking Microtome may be 

 quoted as giving admirable results. 



Various plans, such as coating the edges of the paraffin 

 with softer paraffin, or with Canada balsam, or the employ- 

 ment of specially prepared paraffin, have been recommended, 

 with the idea that they help the sections to stick. None of 

 these devices is necessary. For the prepared paraffin of 

 Spee, Brass, and Fcettinger, see below, 279. 



275. Collodionisation of Sections. Some objects are by nature 

 so brittle that, notwithstanding all precautions taken in 

 imbedding and previous preparation, they break or crumble 

 before the knife, or furnish sections so friable that it is im- 

 possible to mount them in the ordinary way without some 

 impairment of their integrity. Ova are frequently in this 

 case. The remedy for this state of things consists in covering 

 the exposed surface of the object just before cutting each 

 section with a thin layer of collodion, which serves to hold 

 together the loose parts of even the most fragile sections in 

 a wonderfully efficacious way. 



The primitive form of the process was, to place a drop of 



