ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 123 



an abundance of air to supply the animal life under observation. The 

 imprisoned air at once becomes saturated with moisture, as evidenced 

 by the fogginess of the cover ; the central drop cannot evaporate, and 

 the external water will not come in contact with it if care is taken in 

 filling and in adding that lost by evaporation. When not in use, the 

 slide is placed across a small vessel of water, a double and twisted 

 thread arranged in contact with the edge of the square cover, and the 

 whole left for another examination at some future time. 



Nunn's Pillar and other Slides. * — Dr. E. J. Nunn, under the 

 heading of " The Pillar-Slide — a new slide for the Microscope" writes, 

 " Every microscopist knows the difficulty of estimating exactly the 

 amount of fluid which will completely fill the space between a cover 

 and the slide, and consequently a bibulant must be applied to absorb 

 the excess almost always present. This takes a little time, which, to 

 one who has many examinations to make, and who is otherwise 

 pressed, is a matter of some importance. 



The following is a description of a slide intended to obviate this 

 difficulty : — 



Take a small thick cover (round or square, as desired) and cement 

 it on the centre of a slide with Canada balsam. Let this harden 

 thoroughly so that the cover will not slip during warm weather, and 

 also to prevent water insinuating itself between the glasses during 

 the frequent washing to which it will be subjected. Of course it 

 would be better to have these little pillars ground upon the slides, 

 but with care in using them the cemented ones will answer every 

 purpose. 



A drop of the fluid to be examined is placed upon the pillar just 

 described, a cover larger than the pillar is placed upon it, when it 

 will be seen that the excess of fluid flows into the annular space sur- 

 rounding the pillar. Not the least advantage of this new form of 

 slide is that evaporation takes place from the fluid in this annular 

 space, and may go on for a long time without affecting the stratum 

 under examination. 



If desired, the annular space may be filled with oil, and evapora- 

 tion thus be entirely prevented." 



Under the heading of " Chemical— new slide for the Microscope," is 

 the following : — "For the application of chemical tests to fluids under 

 microscopical examination, the ' pillar slide ' presents many advan- 

 tages. The method usual in such cases is to place a drop of the 

 reagent at one edge of the cover and a bit of blotting-paper at the 

 opposite edge, with or without a hair inserted between the cover and 

 the slide to facilitate the inflow of the reagent. 



If the circular pillar-slide be used, then the cover must be pushed 

 so that all the space is on one side ; there will thus be formed a 

 crescentic instead of an annular space. It is evident that in the latter, 

 if the space is filled with reagent it will affect the film, but slowly, 

 because evaporation takes place from the reagent itself, and there is 

 nothing to draw it between the cover and the pillar. In the round 



* Sep. repr. from Trans. Med. Assoc. Georgia, 1883, pp. 21-4. 



