ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



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rather wider, with a central aperture and two side pieces (j in. high), 

 capped with thin strips of brass projecting at either end of the up- 

 rights as shown in the fig. Across the projecting ends two small 

 indiarubber rings are stretched and the slide is passed through these 

 rings and thus suspended. If now the objective is brought down on 

 the slide the latter sinks on the least pressure and ample warning is 

 given to the observer. 



Parsons' Current-Slide. — Mr." P. B. Parsons has devised the new 

 form of current-slide shown in figs. 21 (section) and 22 (perspective), 

 which he describes as follows : — 



" The slide consists of two plates, pierced with central apertures 



Fig. 21. 



surrounded by tubular projections, and fitting together like a live-box. 

 The top one is raised or lowered by a milled head fixed to the lower 

 one and working in a thread cut on the tube of the upper. Two pins 

 prevent the plates from coming apart or turning on each other. 



The top plate has a hole at one end for the water supply and a 



Fig. 22. 



similar hole on the other for the waste, a piece of movable brass tube 

 fitting into each. 



The supply tube has a valve for regulating the quantity of water 

 admitted, and beyond this is an indiarubber pipe connected with the 

 water-vessel. A double-necked bottle is very convenient, so that a 

 fresh supply of any fluid can be introduced without disturbing any- 

 thing. 



The advantages of this arrangement are : — 



1. The depth of the cell is easily adjusted while on the stage, 

 and the object can be brought within reach of fairly high powers by 

 simply reducing the depth of water to a thin film. When not under 

 examination with such powers the cell can be deepened, giving plenty 

 of space with a constant current of fresh water, and yet enabling the 

 observer to keep the object in view with a lower power. 



2. The diameter of the cell, while large enough for all ordinary 



