Microtomes : their Choice and Use 



235 



surfaces of ' rothguss ' (an alloy of zinc and copper) . 

 The two heavy blocks of metal — 0, the object-carrier, 

 and K for the knife — have each five ivory points (three 

 only are seen in the diagram), which slide upon the 

 raised surfaces of the guides in an absolutely perfect 

 manner. Obviously, as is pushed up the incline so the 

 object will be raised. The inclination of the tramway is 

 1 in 20, so that if the object-carrier is pushed up 1 milli- 

 metre the object must be raised ^V oi a millimetre, and 

 the section cut will be of that thickness. To the upper 



Fig. 60. 



surface of the central plate C, Fig. 60, a scale is fitted, 

 reading directly by means of a vernier elevations of the 

 object so small as 0"015 millimetre, or 15 /i ; consequently 

 the object-holder may be pushed up its tramway by hand 

 for sections of this tenuity or thicker. For sections of this 

 or, in fact, any thickness, it is best to use the micrometer 

 screw. This screw is seen in situ to the right of Fig. 59. 

 The working of this micrometer screw requires a little ex- 

 planation. The whole of this apparatus fits into the tram- 

 way, to which it may be fixed at any part by means of a 

 milled-head screw below. The micrometer screw is fitted 



