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NitrATE oF SILVER.—Cornea.—For the investigation of 
the stellate bodies in the cornea, known as ‘‘ Negative images,” 
it is best to use nitrate of silver in the solid form. The 
surface of the cornea, if possible whilst still in situ, must first be 
scraped all over with a scalpel, so as to remove the conjunc- 
tival epithelium. This requires considerable force, as the 
epithelium is closely adherent. ‘The exposed surface is then 
‘rubbed with a piece of caustic for a few seconds, and further 
action of the silver is suddenly stopped by a stream of 
common salt solution (about 2 per cent.). The cornea, which 
should now appear white and almost opaque, should be well 
washed in water and immediately put up in glycerine. If it be 
a small one from a frog, e.g., it can be mounted whole, radial 
cuts being made in it to allow of its lying flat. If it be large 
and thick, small sections parallel to the surface should be 
made with a razor. The stellate bodies will be visible almost 
immediately, but at the present time of the year, when a 
strong sunlight cannot be obtained, from twelve to twenty- 
four hours are necessary to darken the back-ground nicely, 
and throw them properly into relief. 
The writer has thus prepared the cornea of the ox, cat, 
rabbit, pigeon, and various amphibia. The latest account of 
the appearances presented in such preparations will be 
found in a paper by Schweigger-Seidel in the ‘ Bericht der 
mathem. phys. Classé der Konigl. Sichs. Gesellschaft der 
Wissenschaften,’ 1869 (see also the Chronicle in the present 
number of this Journal). 
Stomata in lymph sac of Frog.—For thin membranes a 
solution of nitrate of silver, 3 per cent. is used. The stomata 
in the wall of the large abdominal lymph sac of the frog, 
and which are of great interest, may be thus prepared :—The 
abdomen of a frog just killed is opened, and the intestines are 
raised up; on one side a very fine membrane will then be 
seen stretching across horizontally from the median line to 
the abdominal wall, and forming a sac filled with a perfectly 
transparent fluid. As much of this membrane as possible 
should be removed and instantly plunged into the silver 
solution. It should be held by one corner with forceps, and 
agitated in the solution for about two minutes, so that all 
parts may be equally impregnated. Itshould then be placed 
in distilled water and remain there till it assumes a light 
brown tint, when it should be carefully spread out on a slide 
and mounted in glycerine. The stomata are figured in Kolli- 
ker’s ‘ Handbuch der Gewebelehre,’ p. 602. For a method 
of staining the nuclei of the cells and mounting in Canada 
balsam, see p. 283 of this Journal, vol. x, new series, 
