OF THE MICROSCOPE. 189 



blades so arranged in one handle that their distance from euch 

 other may be easily regulated. When a cut is made with this 

 double-bladed knife, a thin slice of the tissue passes between 

 the blades, and constitutes the section. It is an instrument, 

 however, which will hardly be used by beginners. Sections 

 of substances of greater consistence, such as wood and soft 

 bones, are most easily made in a regular section cutter. The 

 patterns according to which these instruments are constructed 

 are very various, but they all act on the principle of raising 

 above the surface of a brass table, by means of a fine screw, the 

 substance to be cut, and then passing a very sharp razor or 

 knife over the table so as to shave off the projecting part of the 

 object. The table is usually of brass, ground and polished. 

 This gives rise to two serious defects. The metal is too soft in 

 the first place, so that it is impossible to press with sufficient 

 force on the razor without cutting into the table, and secondly, 

 when any soft metal has been .ground on a grindstone or emery 

 wheel, the surface becomes so impregnated with gritty matter, 

 that it very rapidly destroys the edge of the cutting tool. We 

 avoid these difficulties by fitting to our section cutter a stout 

 plate of hardened steel, the surface of which has been highly 

 polished by means of buff leather. Quekett describes a cut- 

 ting machine in which the difficulties we have mentioned are 

 obviated by fixing the knife in a frame so that it is raised above 

 the table, and does not touch the metal. Its edge is thus pre- 

 served from injury, and the blade itself cannot be affected by 

 variations in the pressure exerted. Dr. Curtis, of this city, has 

 adopted the same principle in his section cutter, the details of 

 which are admirably carried out. 



In making sections of wood and similar substances, the speci- 

 men is first well soaked in dilute alcohol, and is then fastened 

 securely into the tube of the section cutter, either by wedges or 

 by casting wax or paraffin around it. The process of raising it 

 by means of the screw and passing the knife over it, is simple 

 enough, and can easily be learned. 



With the ordinary cutting machine, success in making thin 

 sections seems to depend upon the perfect sharpness of the 

 cutting edge, the thorough moistening of the knife and section, 

 ind the rigidity of the blade. The latter point frequently fails 



