INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



715 



fluid coming away, the under plate may be made a very little larger 

 than the upper. 



These plates are turned with the finger about their axis (which is 

 fixed to the stage so as to move from behind forwards, and vice versa), 

 an arrangement which allows the examination of a continuous series 

 of preparations lying in the ijcripheries of different circles 18 to 

 24 cm. in circumference, whilst by the instrument last described a 

 continuous line of only aboxit 3 cm. in length can bo examined. 



When one periphery has been examined (which is indicated by a 

 catch-spring), a rack and pinion moves the plates in a radial direction 

 (as a rule it is best to begin with the inner circle), and they are again 



Fig. (Jo. 



revolved ; and so on until the last periphery has been examined. There 

 is hero the great advantage that in adjusting a fresh circle the size of 

 the field corresponding with tlie power used can be taken into account. 

 Thus tho lowest power recjuires the rack to bo moved three teetli, the 

 medimn jjowcr two teeth, and the highest jiower one tootli forward. 



\Vh(!n tlio plates are r-enioved from the stage the instrument can be 

 used as an ordinary ]\Iicrosc(jj)e. 



Ti'schurs * is a simpler form, of tho design shown in Fig. 63. 



It has a wide inclined stage, on which is a bar attached to two 



supports, after the manner of a parallel ruler. ]5y means of tho 



adjusting screw h, the bar can be moved to and from the aperture 



* 1"'. W. llufltrt, ' MikiObkopiBchc Flcibchboschau ' (8vo, Leipzig, li<yO) j). 51. 



