ZOOLOGY AND BOTANY, MIGROSCOPYj ETC. 



157 



the object. Still more exact is the working of the clamp shown in 

 fig. 39. There is rotation round two horizontal axes, as before, 

 the position of the clamp being fixed at any given point by the two 



Fig. 39. 



handles d and d'. The milled heads / and /' produce the rotation by 

 means of toothed wheels. The jaws ^' are moved by K to fix the 

 specimen to be cut. 



This clamp has received some more recent improvements, which 

 allow also of a vertical movement of the specimen in the clamp, and 

 rotation round a vertical axis. It also allows of the use of imbedding 

 boxes. 



The microtome is now made of non-oxidizable bronze. 



Herr Jung also provides * a strop of large size for the knives. 

 The edges of the leather are carried round beneath to obviate the 

 wrinkling which the tension of the blade usually produces on the sides. 



The placing of the knife in a transverse position is found to 

 prevent to a great extent the curling up of the sections. It also 

 enables the successive sections to adhere to one another, to form a 

 continuous ribbon. 



Cutting Ribbons of Sections. — M. A. Gravis enumerates f several 

 conditions which are necessary to the success of this delicate process. 

 The object must not be too large nor too friable ; the melting-point 

 of the paraf&n must be chosen with great precision, and the temjjera- 

 ture of the room must be in a certain relation to that of the melting- 

 point of the paraffin. A hard paraffin is favourable for the thinnest 

 sections ; and a soft paraffin facilitates the adherence of the sections 

 in ribbons. Mr. Harmer reconciles these two opposite advantages by 

 using as hard a paraffin as the nature of the object permits ; cutting 

 out of the block of paraffin a small cube containing the object, then 



* Bull. Soc. Belg. Micr., x. (1884) pp. 151-2. 

 t Ibid., pp. 117-9. 



