INVEETEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



881 



pointer /on the scale of/'. Suppose a Pleurosigma attenuatum of a 

 certain preparation to lie in the field when the pointer / was on the 

 division 18 of the scale /', and it is required to find this frustule 

 again. Solution : Put /on the division 18 of/', place the preparation 

 firmly between the clanij)S E E, and turn the plate C about D, the 

 Pleurosigma sought for must iacvitably appear in the field. 



A great advantage of this stage consists in its enabling the 

 observer, by simply taking out the screw H, to move the preparation 

 about in any direction under the objective, just as in the case of free- 

 hand movement of the object with the ordinary stage. 



The second stage (Fig. 96), whicli is the more complicated, and is 

 intended for scientific investigations and for measuring, is similar in 

 construction to the stage above described. 



Fig. 96. 



The plate A is dovetailed into the microscope-stage B, and by 

 means of the divided screw C can be moved longitudinally. It has a 

 scale at A, the divisions on which correspond to a revolution of the 

 screw C (0 ■ 25 mm.). The drum of the screw C is divided into one 

 liundrcd parts, each division having thus the value 0*0025 mm. 

 Ihere is also a nonius which marks the tenth part of this value. 



On the plate A there is a second movable plate (on which is an 

 cxccntric disk E, and the i>iece F for fixing the slide), movable about 

 the screw D by the pinion at G, so that one minute may bo read oft' 

 directly by means <.f the scale and nonius. 



It is not necessary to prove that the same jirinciple is involved in 

 the construction of both stages, and consequently that with the second 

 stage an equally systematic, and indeed much more perfect reading ofif 

 of the position of the object is possible, as with the first stage. It can 

 also I e used in just the same way as a IMaltwood finder, but giving, of 



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