CHAIN OK " RIBBON " SECTION-CUTTING. 183 



to be the case, the knife should be readjusted by means of a 

 piece of cardboard placed in the jaws of the clamp. I would 

 suggest that another cause of these defects is to be found in 

 imperfection of the edge of the knife. If the knife be blunt, 

 or have a rounded or curled edge caused by untrue honing or 

 stropping, it will of course act in respect to minute structures 

 as a plough rather than a cutting instrument, and thus produce 

 the appearances described. 



281. Chain or " Ribbon " Section-cutting. If a series of 

 paraffin sections be cut in succession and not removed from 

 the knife one by one as cut, but allowed to lie undisturbed 

 on the blade, it not unfrequently happens that they adhere 

 to one another by the edges so as to form a chain which may 

 be taken up and transferred to a slide without breaking up, 

 thus greatly lightening the labour of mounting a series. The 

 following appear to me to be the factors necessary for the 

 production of a chain. 



Firstly, the paraffin must be of a melting-point having a 

 certain relation to the temperature of the laboratory. Small 

 sections can always be made to chain when cut from a good 

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

 mometer stands at 16 to 17 C. (The temperatures quoted 

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

 bably 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 that can be produced. For it is the 

 sharp impact of the knife that slightly heats, and therefore 

 slightly softens the near edge of the paraffin, and thus causes 

 the sections to cohere. It is evident that this condition can 

 only be conveniently realised by means of a sliding micro- 

 tome ; but it is by no means necessary to have recourse to 

 special mechanical contrivances, as in Caldwell's automatic 



