CUTTING THIN SECTIONS. 189 



— Where only a moderate degree of thinness is required, either 

 in consequence of the transparence of the tissue, or because 

 it is not desired to exhibit its minutest details, the two-bladed 

 Knife contrived by Prof. Valentin (Fig. 94) may be employed 

 with advantage. The blades are attached to each other at their 

 lower end by a screw, in such a manner that their ' spring ' tends 

 to keep them apart ; and their distance is regulated by pushing 

 the little rivet backwards or forwards in the slit through which 

 it works. The knife should be dipped in water before using, or, 



Fig. 94. 



Valentin's Knife. 



still better, the section should be made under water, as the instru- 

 ment works much better when wet ; after use, it should be care- 

 fully washed and dried, a piece of soft leather being passed 

 between the blades. If any water have found its way into the 

 part through which the rivet works, the movable blade should be 

 detached by taking out its screw, and each blade should be cleaned 

 separately. * 



137. Cutting Sections of Harder Substances. — There is a large 

 class of substances, both Animal and Vegetable, which are too 

 hard to admit of sections being made in the manner just described, 

 but of which extremely thin slices can be made by a sharp cutting 

 instrument, if only they be properly held and supported, — more 

 especially when the thickness of the section can be regulated by a 

 mechanical contrivance ; such are, in particular, the Stems and 

 Roots of Plants, and the Horns, Hoofs, Cartilages, and similarly 

 firm structures of Animals. Various costly machines have been 

 devised for this purpose, some of them characterized by great 

 ingenuity of contrivance and beauty of workmanship ; but every 

 purpose to which these are adapted will be found to be answered 

 by a very simple and unexpensive little instrument, which may 

 either be held in the hand, or (which is preferable) may be firmly 

 attached by means of a T-shaped piece of wood (as in Fig. 95), to 

 the end of a table or work-bench. This instrument essentially 

 consists of an upright hollow cylinder of brass, with a kind of 

 piston which is pushed from below upwards by a fine-threaded 

 screw turned by a large milled-head ; at the upper end the cylinder 

 terminates in a brass table, which is made to present a perfectly 



* An improved form of this instrument is constructed by Mr. 

 Mathews of Portugal-street ; the blades being made with a convex 

 instead of a straight edge, their distance from each other being regulated 

 by a milled-head screw, and their separation for cleaning being more 

 easily accomplished. 



