ZOOLOaY AND BOTANY, MICEOSCOPY, ETC. 



843 



Fig. 198. 



form a biconcave surface. Its total length is 64 cm. or deducting the 



two handles 44 cm., and its breadth 4*5 cm. The thickness of the 



back is 1'5 cm., and its weight 2062 grammes. 



The two solid fixed handles of lead are each 



1 decim. long, and can be conveniently grasped 



with the whole hand. Lead is employed to 



increase the weight of the instrument. 



The force with which the edge of the knife 

 is applied to the object is obviously equal to 

 the product of the weight of the knife and the 

 velocity of its motion. It takes a very short 

 time to learn how to manipulate a heavy knife 

 with as much ease as a light one, and, in general, 

 the heavier the knife the easier it is to cut with. 

 For this reason, microtomes which are adapted 

 for heavy knives are far preferable to those 

 which, like the Eivet and its modifications, can 

 only be worked with thin knives. 



It has been urged against the principle of 

 heavy knives that many animal objects are much 

 too delicate for them, that by such large cutting 

 instruments friable preparations can indeed be 

 crushed, but not cut. This is not so. It is 

 simply necessary to harden the preparation pro- 

 perly (which can be effected perfectly by the 

 method described in the preceding note), and 

 any object, however delicate or brittle, will not 

 be affected by the weight of the knife. There 

 can certainly be nothing more difficult to cut 

 than the ova of Rana or Bufo, and yet, when 

 these are properly hardened and well permeated 

 with Klebs' substance, they can be beautifully 

 cut with the largest knife. Sections of embryos 

 of rabbits, 3 mm. long, are also cut, notwith- 

 standing such objects are most delicate. 



Making Sections very quickly.* — H. de 



Lacaze-Duthiers recommends, for obtaining 

 sections very quickly for preliminary inves- 

 tigation, that the object should be put into a 

 strong, dark yellow, very hot solution of chromic 

 acid. In a few minutes it will be so far 

 hardened that the spot which is to be cut can 

 be isolated ; the latter (e. g. a small ganglion, &c.) is then transferred 

 to a weak light yellow chromic acid solution of the ordinary tem- 

 perature, and there left until any other preparations which may 

 have to be made from the animal are completed. 



As imbedding material, glue is used, which must be of the best 

 quality, i. e. quite transparent ; it is used as prepared in two different 



* Axch. Zool. exper. et gen, vi. p. xxxviiL See Zool. Jahiesber., i. (1880) p. 35. 



