168 SUMMARY OF CURRENT RESEARCHES RELATINO TO 



inclination may be imparted. The sections are made automatically 

 and are of a definite thickness. By working to and fro the handle, 

 which is in connection with the microtome screw, and having pre- 

 viously put the spring in action, the machine works automatically, 

 cutting, at each complete turn, a section 1/100 of a millimetre in 

 thickness. Thicker sections are made by stopping short just before 

 cutting and reversing the action of the handle, &c. ; thus two descents 

 of the knife-carrier produce a section U • 02 mm. in thickness, and so on. 



In cutting under water or alcohol the instrument is placed, as will 

 be seen from fig. 43, in a position at right angles to that of fig. 42. 

 A trough full of water or alcohol receives the object-carrier, and the 

 sections fall off into the fluid. 



When used for freezing, the object-grip or tube is replaced by a 

 plate (fig. 42) beneath which is a reservoir for saving the superfluous 

 ether. In place of ether Prof. Malassez advises the use of methyl 

 chloride, which being volatile at the ordinary temperature and 

 pressure, does not necessitate the use of a spray apparatus. A tin 

 tube covered with caoutchouc and fitted with a stopcock is attached 

 to the siphon which contains the methyl chloride. One jet of vapour 

 is nearly always sufficient to freeze the object, and when this is 

 effected it is advisable to place the machine in the vertical position 

 and allow the sections to drop into a basin of water recently boiled , 

 or slightly alcoholized, in order to get rid of air-bubbles. 



Sharpening Microtome Knives.* — Dr. C. O. Whitman considers 

 that microtome knives can be properly sharpened only by those who 

 understand their chief peculiarities, and who have trained them- 

 selves in this special work. The difficulties in acquiring the art 

 are not, however, insurmountable ; for with 

 ^^^- '*^- the proper means and a little perseverance 



they can be mastered in a short time. The 

 first important step is to provide oneself 

 either with a good razor-strop, or with a 

 long and wide oilstone of the finest quality. 

 Strops made of a leather band, unsupported by a solid base, and kept 

 tense by the aid of a screw working in a frame, should never be 

 employed in sharpening these knives, for they invariably give a bi- 

 convex edge, with which it is impossible to do fine work. To secure 

 a plane bevel of the cutting edge the surface of the strop must be 

 perfectly smooth, flat, and hard. In using the strop the knife is 

 drawn back and forth, back foremost, without pressure, until the edge 

 appears sharp when tested in the manner before mentioned. 



In using an oil-stone it is well to cover the surface of the stone 

 with a mixture of glycerin (two parts) and water (one part). The 

 blade is laid flat on the stone and pushed forward, edge loremost, in 

 such a manner that the free end of the knife finishes by resting on 

 the more distant end of the stone. Here the blade is turned on its 

 back and returned, edge in advance as before, to the place of starting. 

 In drawing the blade the utmost care should be taken never to raise 



* Amer. Natural., rix. (1885) pp. 831-2 (1 fig.). 



