PARAFFIN METHOD 89 



long axis, and though more costly, will be found a great conveni- 

 ence where much section-cutting has to be done. 



168. Safety Razor Blade Holders. We have used one of these 

 supplied by Leitz. It performed quite well, and is a solution of 

 the difficulty of providing microtome kniv^es for classes of students. 



169. Cutting and Section-stretching. Paraftin sections are cut 

 dry, — that is, with a knife not moistened with alcohol or other 

 liquid. By this means better sections are obtained, but a difficulty 

 generally arises owing to the tendency of sections so cut to curl 

 up on the blade of the knife. It is sometimes difficult by any 

 means to unroll a thin section that has curled. To prevent 

 sections from rolling, the following points should be attended to. 



First and foremost, the paraffin must not he too hard, see § 159. 



If, after cutting has begun, the paraffin be found to be too hard, it 

 may be softened by placing a lamp near the imbedded object. But 

 then, the paraffin being warmed most on the side nearest the lamp, 

 becomes softer on that side, and the sections have a tendency to become 

 compressed and puckered-in on that side. 



If, on the contrary, the paraffin be found too soft, it may be hardened 

 by exposing it to the cooling influence of a lump of ice. 



It is often sufficient to moderate the temperature of the room by 

 opening or closing the window, stirring the fire, setting up a screen, or 

 the like. 



For other devices for warming or cooling the paraffin see Held, Arch. 

 Anal. Phys., Anat. Abth., 1897, p. 345 ; van Walsem, Zeit. wiss. Mik., 

 xi, 1894, p. 218 ; Lendenfeld, ibid., xviii, 1901, p. 18 ; Krause, ibid., 

 XXV, 1908, p. 299 ; Foot and Strobell, Biol. Bull. Wood''s Hole, ix, 

 1905, p. 281. 



Secondly, the knife should be set square, for the oblique position 

 encourages rolling, and the more the knife is oblique the more do 

 the sections roll. 



Thirdly, it is better to cut ribbons than disconnected sections ; 

 ribbons of sections will often cut Hat, when the same mass will 

 only give rolled sections if cut disconnectedly. 



Rolling may often be lessened or suppressed by cutting the 

 sections thinner. 



Mechanical means may be employed. The simplest of these is 

 as follows : 



During the cutting the edge of the section that begins to curl 

 is caught and held down on the blade of the knife by means of a 

 small camel-hair brush with a Hat point, or by a small spatula 

 made by fixing a piece of ])aper on to the back of a scalpel. 

 Or, which is much lietter, the section is held down by means of an 

 instruinent called a " section-stretcher." This consists essentially 

 of a little metallic roller suspended over the object to be cut in 

 such a way as to rest on its free surface with a {)ressure that can 

 be delicately regulated so as to be sufficient to keep the section 

 flat without in any way hindering the knife from gliding beneath it. 



