THE THOMA MICROTOME 467 



1 uiiid: but it is much more accurate to control its movement with 

 the micrometer-screw. The point of this screw in fig. 392, t, works 

 on the polished plane of an agate cone. The clamp on which the 

 screw is mounted is held firmly in its place by the milled head "VV in 

 ftch. It may stretch up as far as O, being refastened by W. 



The screw m is so cut that a single rotation moves the slide 011 

 the y,f' { ," - mm., which in the inclination of the plane of 1 : 20 gives 

 an elevation of the object of i}^ mm. The barrel or drum, 'K, 

 situated on the axis of the screw, is divided into fifteen parts ; con- 

 sequently the interval of each division corresponds to an elevation of 



nAnrmm-. 



There is also an action by means of a spring which gives the ear 



as well as the eye cognisance of tne amount of elevation which has 

 taken place, which greatly relieves the eye. This,, however, can be 

 brought into action or not at the option of the operator. 



Besides these object-holders a freezing apparatus can be added 

 which is simply placed on the object-slide as shown in fig. 397. 



FIG. 397. Freezing apparatus for the Thoma microtome. 



The freezing is effected by ether-spray. A specially favourable 

 effect is obtained if the cylinder g is mica and not glass. A layer of 

 water freezes in from thirty to thirty-five seconds. 



An arrangement of the Thoma for cutting large objects has also 

 been devised which is illustrated in fig. 398. 



The knife is to be placed considerably higher in front than 

 behind, in order to lessen the pressure on the objects. In order to 

 satisfy all demands, the knife-rest is adjustable. 



The knife is so arranged that the whole length of blade can be 

 used, and then the screw c is fairly tightly screwed down. As strong 

 knives, even of a length of 36 cm., easily give, a knife-support has 

 been constructed ; this is fastened by the screw c' to the carrier. 

 The support is arranged parallel with the back of the knife M ; if 

 the extremity n be slightly pressed backwards, so that it touches the 



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