862 RECORD OF CURRENT RESEARCHES RELATING TO 



itself is not so mucli injured, but the covering glass will, sooner or 

 later, become covered (inside tlie cell) on the under surface, with a 

 dew-like deposit, which, when illuminated, will glisten almost like so 

 many minute points of quicksilver, and though out of focus when the 

 object is viewed, will show very disagreeably, like a thin gauze 

 between ; and with transparent objects these minute globules will not 

 only dot the entire field as so many dark or light points, but the 

 object itself will appear as though it had been wetted. 



Not long ago a well-known optician showed to me a spoiled slide 

 oiPodura. The scales were very good and large — in fact, it was a slide 

 which I had given to him, and it had been selected by myself in 

 Beck's establishment in London as unexceptionably fine. This slide 

 began slowly to show symptoms of ' sweating.' One scale after 

 another appeared, as though moisture had, in some mysterious way, 

 penetrated to the objects ; it was not water, however, for when the 

 cover, after much trouble, had been removed, and warmed sufficiently 

 to evaporate anything like water, the scales still exhibited the same 

 appearance, and, in fact, the heat required to get rid of this apparent 

 moisture was so great that the scales were charred. When wax rings 

 are used, this apparent wetting or 'sweating' occurs quickly, and 

 more disagreeable than this, innumerable elongated sj)ecks, possibly 

 crystalline, appear all over the under surface of the cover-glass. 

 The same trouble occurs when any of the ordinary asphalt prepara- 

 tions are used, and the only cement which I have thus far found to 

 be tolerably successful is shellac thoroughly incorporated with the 

 finest carbon (diamond black), such as is used in the preparation of 

 the best printing inks ; the solvent being alcohol, these rings dry 

 rapidly, and the cover is attached by heating. Even these rings 

 cannot be trusted, unless thoroughly dry, and siiontaneous drying is 

 better than baking. I have had preparations spoiled after mounting 

 on asphalt rings, which had been made for over a year, and which 

 had been subjected for several hours to the heat of a steam-bath. 

 With large, somewhat coarse objects, the defect is not so marked ; 

 but with delicate ones, and especially test objects, it is simply a 

 nuisance. With care I think the shellac rings may answer pretty 

 well. I have not tried the aniline coloured rings. The moisture 

 (whatever it is), and the crystalline specks, appear to be derived from 

 the vaporizable parts of the wax, or cement, given off under con- 

 ditions where one would suppose such a thing impossible ; it is, how- 

 ever, a fact ; I have the proof of it, and I dare say hundreds of others 

 have, too plainly evident. 



There is another mode of making cells which promises well for 

 permanence. My attention was first called to this method by Dr. Tulk, 

 of London, who suggested for this purpose the thin gutta-percha tissue 

 used by surgeons in the place of oiled silk. I have had special 

 punches made, which cut neat rings from this tissue, and I have used 

 these rings with the greatest satisfaction. I have no preparation of 

 my own more than about two years old ; these, so far, show no signs 

 of change. Dr. Tulk informs me that he has them ten years old, and 

 still good as when new. I have noticed that in some recent papers 



