934 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the horizontal arm, and vertical motion is provided by rackwork on 

 the column. The rectangular motions give a moderate range within 

 which a moving object can be kept in view. 



The above arrangement is not, however, so convenient as the 

 " Eoss Tank (or " Aquarium and Physiological ") Microscope," shown 

 in Fig. 204, for examining large objects of all kinds. There is a 

 vertical movement on the upright standard and lateral and inclining 

 movements on the horizontal bar (the two former by rack and pinion). 

 The two small milled heads at the side of the standard give rectan- 

 gular motions to the body-tube of the Microscope, and the large one on 

 the left is for focussing. A convenient table-stage with mirror (not 

 shown in the figure) can be used with the instrument, or the table- 

 tank shown under the monocular stand. 



Fig 205 



Nachet's Petrographical Microscope. —This instrument, de- 

 scribed by M. Nachet at p. 227 of vol. iii., has been further elaborated 

 by him, and is now issued in the form shown in Fig. 205. 



