CUTTING THIN SECTIONS. 205 



section taken from a flat surface will generally be found to .thin 

 away, although the middle of it may he too thick to exhibit any 

 structure. Where only a moderate degree of thinness is re- 

 quired, 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. 58) 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 regu- 

 lated by pushing the little rivet backwards or forwards in the 



FIG. 58. 



Valentin's Knife. 



slit through which it works. The knife should be dipped in 

 water before using, or, still better, the section should be made 

 under water, as the instrument works much better when wet ; 

 after use, it should be carefully 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. 1 



107. 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 de- 

 scribed, but of which extremely thin slices can be made by a 

 sharp cutting instrument, if only they be properly held and sup- 

 ported, 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 ma- 

 chines have been devised for this purpose, some of them charac- 

 terized by great ingenuity of contrivance and beauty of work- 

 manship ; 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. 59), to the end of a table or work-bench. 



but towards the observer ; these scissors being, as the French instrument-makers say, 

 courbes sur le plat. As an example of the utility of such an instrument to the Micro- 

 scopist, the Author may cite the curious demonstration given a few years since, by Dr. 

 Aug. Waller, of the structure of the gustative papillae, by snipping oft" the papillae from 

 the living human tongue, which may be done with no more pain than the prick of a pin 

 would occasion. 



1 An improved form of this instrument is constructed by Mr. Matthews 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 clean- 

 ing being more easily accomplished. 



