108 . CHAPTER VIII. 



sided prism, and orientated as in the first case^ so that the 

 knife first touches one ang'le, if the sections are to be cut 

 singly. But if ribbons are to be cut, the block must be 

 orientated with one of its sides parallel to the knife-edge, 

 and the opposite side must be strictly parallel to this one. 



For NOACK'S simple apparatus for accurately orientating 

 small blocks, see Zeit. f. wiss. Alik. xv, 1899, p. 438, or 

 Journ. Roy. Mic. Soc., 132, 1899, p. 550. 



For ETERNOD'S machine for trimming blocks to true cubes, see Zeit.f. 

 wiss. Mlik., xv, 4, p. 421, or Journ. Roy. Mic. Soc., 1899, p. 450. 



135. Cutting and Section-stretching. Paraffin 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 be^top_Jiard, but 

 must be taken of a melting-point suitable to the temperature 

 of the laboratory, see 111. 



The exact degree of hardness necessary must be deter- 

 mined by experiment. If, after cutting has begun, the 

 paraffin be found to be too hard, it may be softened by 

 placing a lamp, or any convenient source of heat, near the 

 imbedded object. I find that a mere spirit lamp set up 

 near the object will sometimes bring the paraffin to the 

 right consistency in a few minutes. 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 placed in the focus of a parabolic reflector. 



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. 



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. Not that a square-set knife 



