2nd. I believe collodion will keep good much longer than is generally supposed; at the beginning of last month I obtained a tolerably good portrait of Mr. Pollock from some remains in a small bottle brought to me by Mr. Archer in September 1850; and I especially notice this fact, as it is connected with the first introduction of the use of collodion in England. Generally speaking, I do not find that it deteriorates in two or three months; the addition of a few drops of the iodizing solution will generally restore it, unless it has become rotten: this, I think, is the case when the gun cotton has not been perfectly freed from the acid. The redness which collodion assumes by age, may also be discharged by the addition of a few drops of liquor ammoniae, but I do not think it in any way accelerates its activity of action. 3rd. "Washed ether," or, as it is sometimes called, "inhaling ether," has been deprived of the alcohol which the common ether contains, and it will not dissolve the gun cotton unless the alcohol is restored to it. I would here observe that an excess of alcohol (spirits of wine) thickens the collodion, and gives it a mucilaginous appearance, rendering it much more difficult to use by its slowness in flowing over the glass plate, as well as producing a less even surface than when nearly all ether is used. A collodion, however, with thirty-five per cent. of spirits of wine, is very quick, allowing from its less tenacious quality a more rapid action of the nitrate of silver bath. 4th. Cyanide of potassium has been used to re-dissolve the iodide of silver, but the results are by no means so satisfactory; the cost of pure iodide of potassium bought at a _proper market_ is certainly very inconsiderable compared to the disappointment resulting from a false economy. H. W. DIAMOND. Surrey County Asylum. _Trial of Lenses._--When you want to try a lens, first be sure that the slides of your camera are correctly constructed, which is easily done. Place at any distance you please a sheet of paper printed in small type; focus this on your ground glass with the assistance of a magnifying-glass; now take the slide which carries your plate of glass, and if you have not a piece of ground glass at hand, insert a plate which you would otherwise excite in the bath after the application of collodion, but now _dull_ it by touching it with putty. Observe whether you get an equally clear and well-focussed picture on this; if you do, you may conclude there is no fault in the construction of your camera.