76 JOURNAL OF THE [May, 



until all is dissolved. This mixture when cold should be very 

 viscid. 



In making a preparation with this medium, at first, on heat- 

 ing, a great number of small bubbles may appear under the 

 cover. A little more heating enlarges these to steam-bubbles, 

 then, by allowing the slide to cool a little, the cover will settle 

 down, and most of the bubbles will disappear ; but if any are 

 still present, another application of the heat of a small flame 

 under the slide at the edge of the cover, where the bubbles are 

 most abundant, will remove them. Towards the completion of 

 the preparation, the slide may be inverted, if necessary, and the 

 small flame allowed to play directly on the edge of the cover ; 

 thus, careful treatment will dispose of all bubbles. When 

 cold, the excess is easily removed with a moistened roll of tissue 

 paper, and, finally, after the cleaning, the slide should be warmed 

 just sufficiently to expel any moisture that may have found its 

 way under the cover. If, after the ring is applied and the prep- 

 aration otherwise completed, any metallic stain should show 

 on the cover or slide, it can be removed with a roll of tissue 

 paper moistened with hydrochloric acid. 



The arsenious acid also makes an excellent compound with 

 the antimony bromide ; and the highest-refractive-power white 

 medium that I have yet seen is made as follows : Melt anti- 

 mony bromide and add to it while hot half its bulk of glycerine ; 

 in this put arsenious acid, little by little, shaking and heating at 

 the same time, until by its solution the bulk is increased three- 

 fourths of one part, so that the final mixture will be : antimony 

 bromide 2 parts, glycerine i part, arsenious acid % part, all in 

 bulk. This compound is solid, or very nearly so, when cold, 

 and will require slight warming to take out a drop on the dip- 

 ping-rod. It does not soften much, if at all, on exposure, and 

 its refractive index is well on towards 2. The mounts made 

 with this material are very satisfactory. 



Finally, I think that the yellow medium, the compound of 

 "realgar" and bromide of arsenic, can be made permanent and 

 easy to use by the addition of a small excess of sulphur. The 

 realgar is broken up and dissolved, by the aid of heat, in the 

 bromide of arsenic. The solution is evaporated until, when 

 cold, it becomes so viscid as to flow with difficulty ; enough 

 gulphur is now added to increase its bulk about one-sixth (I 



