132 The Microscope. 



great quantities at once, it will not pass through the opening in the 

 inner ring, thus leaving an abundance of air to supply the animals 

 under observation. The imprisoned air at once becomes saturated 

 with moisture, as evidenced by the fogginess of the cover ; the 

 central drop cannot evajjorate, and the external water will not come 

 in contact with it if care be taken in filling the slide and in supply- 

 ing that lost by evaporation. To admit entirely fresh air the water 

 can be drawn off by bibulous paper, or allowed to evaporate, without 

 in any way disturbing the central drop. The reader must remem- 

 ber, however, that it is intended for only the smallest of microscopic 

 animals, with which it is entirely successful. 



I think it was in the proceedings of the Cleveland meeting of 

 the American Society of Microscopists that Mr. J. H. Logan de- 

 cribed an excellent life-slide which I have used a good deal and 

 successfully, but as it has some characterestics that render it scarcely 

 adapted to my needs, I have in a rough way made, on Mr. Logan's 

 principle, an imitation which answers my purpose better than the 

 more ornate slides supplied by the inventor. My fi-iend Dr. T. S. 

 Stevens of this city (Trenton, N. J.) tells me that he has kept Eoti- 

 fers for a whole month alive and well in Mr. Logan's slide, the 

 Rotifers {Furciilaria) depositing eggs and apparently flourishing as 

 if in entire fi-eedom. For my own purpose, however, the slide is 

 objectionable because it is so hea^y, the central disc is so ex- 

 tremely thick that no sub-stage condenser can be focussed through 

 it, and the .annular depression so deep that the glass sides afPect the 

 light in an undesirable way. For comparatively large aquatic 

 objects to be examined with a low power it is admirable, but I find 

 my own modification of it better for my own work. A small square, 

 cu.t from glass of any desired thickness, is cemented with Canada 

 balsam to a slip, and surrounded by a thick glass or zinc ring so as 

 leave a wide space between these parts. On the ring place a ring of 

 wax and, after the object has been arranged on the central square, 

 cover the whole with a thin circle and cement it fast by running a 

 warm wire around the edge to melt the wax. A small drop of water 

 may be placed in the annular space if desired. The reader of coiu'se 

 understands that the thickness of the slip and square, and the depth 

 of the cell must be determined by each worker according to his 

 needs. For myself I have them as thin and shallow as possible. 

 [A diagram of the contrivance is shown in fig. 3.] The secret of 

 success here is, to be sure, that the joint between the ring and the 

 slip is air-tight, and to firmly secure the cover, using an abundance 

 of wax. 



