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XVII. — New Tank, and Fond-iueed Holder, to be used with the 



Greenough Microscope. 



By Sydney C. Akehurst, F.R.M.S. 



{Read November 15, 1916.) 

 Plates XVIII-XIX. 



The tank is all metal, with an aperture ab the bottom 1| in. by |in. 

 Over this is cemented a cover-glass, which works clear of the stage, 

 the base being counter-sunk to allow this. The height of the front 

 is 1^ in., the back 2 in. This variation admits of it being used on 

 the Microscope when inclined. The width is 1| in., and the base 

 3 in. by 1| in. The sides slightly taper to accommodate the shape 

 of the paired objectives, and allow working to the edge of the field. 

 The fluid capacity is 3 oz. 



Water to be examined is poured into the tank, and, as most 

 free-swimming organisms are phototropic, they soon gather in the 

 field of view, and an opportunity is given of watching them in all 

 aspects, as there is ample room for movement. Owing to the wide 

 mouth of the tank, a comparatively large surface of the water is in 

 contact with air, and it is possible to retain pond-weed in the 

 water for the purpose of aeration without obstructing any part of 

 the field. Organisms can, therefore, be kept in a vigorous condition 

 for prolonged observation. 



The contents of the receptacle can either be removed or added 

 to, and there is ample working space to take out any object of 

 interest with a pipette whilst the objectives are in focus. Any 

 free-swimming forms slow to react to light can be confined within 

 a limited area in a well at the bottom of the tank, over which a 

 cover-glass is placed. 



The weed-holder hangs by two hooks from the front of the tank. 

 It has universal movement, which admits of any fixed form on pond- 

 weed being viewed in various positions. When necessary to change 

 the weed, the holder can be lifted from the tank without disturbing 

 any other part of the apparatus. 



Easy access to fixed or free-swimming organisms whilst under 



EXPLANATION OF PLATE XVIII. 



1. — Weed-holder, front view, consisting of six separate pieces. 



2. — Ditto, side view. 



3. — View of Tank, with holder in position. 



