240 



Modern Microscopy 



that there is no necessity to unwind it. The continuous 

 working is brought about as follows : When the screw has 

 reached the limit of its movement, it is turned through 

 180°, so that its opposite extremity points towards the 

 object-holder when it is ready for contiiiuing the work. 

 The two improvements to the Eivet model are undoubtedly 

 good in principle, but Herr Eeiehert's improvement to the 

 screw is not entirely advantageous. After turning the 

 screw, the object-holder has to be pushed down the slide 

 until it again touches the point of the screw ; consequently 



Fig. 63. — Minot-Zimmekmaxn Mickotome, without Knife or 

 Objeot-holdee. 



a space will arise between the surface of the object and the 

 knife-edge. This has to be compensated for by raising the 

 object jaws by rack and pinion. Herein lies the weakness 

 of this feature. Every time the rack and pinion has to 

 be brought into action to elevate the object, the ribbon of 

 sections is liable to be broken, and sections are lost before 

 the new ribbon can be started. 



Leitz's Microtomes. — E. Leitz, the optician of Wetzlar, 

 makes a good microtome, moderate in price, and essentially 

 the same as one by Eeichert on the Schanze model. 



