124 



SUMMARY OP CURRENT RESEARCHES RELATINQ TO 



pillar this is best corrected by having the diameter of the cover smaller 

 than that of the pillar, and pushing it to one side so as to project a 

 little beyond the pillar, the lunate space thus formed is filled with 

 reagent, while the rest of the edge of cover is evaporating and drawing 

 upon the reagent to supply the deficiency thus created, or, to hasten 

 the reaction, a bit of blotting-paper may be applied in the usual way. 



Another good vp^ay is to use a square cover : let one of the corners 

 project beyond the pillar, and under this corner put the drop of 

 reagent, in this wslj nearly the vs^hole of the edge of the cover will be 

 left free for evaporation, and the rapidity of the reaction will of 

 course be proportionately great. If desired, a different reagent may be 

 placed under each of the four projecting corners of the square cover. 



The ' square pillar-slide ' seems, however, best adapted to this 

 class of work, with a cover the same size or smaller than the pillar, 

 and projecting a little beyond it ; the reagent will then occupy one 

 side of the square and evaporation go on from the other three sides. 

 If an oblong cover is used which projects on opposite sides of the 

 pillar, then the same or different reagents may be placed on opposite 

 sides of the same specimen, without danger of mixing with each other." 



Under " Slides with hollows for chemical reactions " Dr. Nunn 

 gays " Many of the advantages of the pillar slides for the observa- 

 tion of chemical reactions may be obtained by using polished glass 

 slides with one or more hollows. 



In using these the drop of fluid to be examined is placed by the 

 side of the hollow, or between them, if there be two or more, and the 

 cover is allowed to project over the hollow or hollows a little distance; 

 under this projecting edge the drop of reagent is placed, and the bit 

 of blotting-paper may be used as usual upon the slide if desired." 



Beck's Condenser with two Diaphragm-plates. — Fig. 24 shows 

 the condenser which accompanies Messrs. Beck's Pathological Micro- 

 scope (Vol. III. 1883, p. 894). The 

 peculiarity of its construction is that 

 it has two rotating diaphragm-plates, 

 one with the usual series of (7) aper- 

 tures of different sizes, and the other 

 with one clear aperture and three 

 others filled with blue glass of vary- 

 ing tints, for moderating the light. 

 The former is placed at a distance 

 below the lenses sufficient for accu- 

 rate centering of the condenser. 



As shown in fig. 24, the con- 

 denser is for use with the smaller 

 stands, but by reversing the optical 

 combination and screwing it on the 

 opposite side it is available for large 

 stands. 



By the removal of the sliding cap which carries the highest power 

 lens of the three of which the optical combination is composed, the 

 condenser is suitable for use with low-power objectives. 



Fig. 24. 



