8 INTRODUCTORY. 



time very brittle. The brittleness is, however, sometimes very 

 helpful in minute dissections. Another property of clove oil is that 

 it does not easily spread itself over the surface of a slide, but has a 

 tendency to form very convex drops, and this also makes it frequently 

 a very convenient medium for making minute dissections in. 



If it be desired to dissect in a watery fluid, such as glycerin, it may 

 be well to prepare the slide by spreading on it a thin layer of MAYER'S 

 albumen, and on this place a small drop of glycerin, or other dissect- 

 ing medium. As soon as the dissection has been accomplished, a 

 cover may be let fall, horizontally, on to the preparation to keep the 

 parts in place, and a weight placed on it. Then the mount may be 

 filled up with glycerin, or other mounting medium, run in under the 

 cover, and closed, if desired, or instead of the albumen a solution of 

 gelatin may be taken, and hardened in formol with the objects 

 on it. For a balsam mount, after clove or cedar oil, SCHALLIBAUM'S 

 collodion may be taken, and the organs fixed in situ on this by 

 adding xylol. 



10. Instruments. For all that concerns the mechanism and 

 manipulation of the Microscope, see vol. i of CARPENTER'S The 

 Microscope, eighth edition, 1891 ; paying particular attention to all 

 that is said concerning the English and the Continental Models, 

 pp. 254 to 261, the Substage, pp. 184 to 189, Condensers, pp. 289 to 

 316, and Tube Length, pp. 158 to 159. 



For information concerning the principles of construction and 

 manipulation of the Microtome, see also CARPENTER'S The Micro- 

 scope. Microtomes are instruments for the accurate production of 

 thin slices of tissues. They are used both for cutting tissues that 

 have acquired a certain favourable consistency through having been 

 imbedded in paraffin, and also for cutting tissues that have been 

 imbedded in softer masses, such as collodion, and tissues that have 

 not been imbedded at all. Not all microtomes are equally well 

 adapted for all these three classes of work. The microtome of the 

 zoologist should at all events be one that is well adapted for cutting 

 imbedded material. 



Now there are two methods of imbedding in general use the 

 paraffin method and the celloidin method. In the paraffin method 

 the object is cut dry, frequently with the knife set square to the line 

 of section. In the celloidin method, as in the cutting of unimbedded 

 tissues, it is generally cut wet, and always with the knife set slanting. 

 Some microtomes that are well adapted for the paraffin method are 

 ill adapted for the celloidin method or the cutting of unimbedded 



