The following is a description of processes for obtaining photographic pictures of landscapes on paper, as practiced by J. Stewart, brother-in-law of Herschel, the astronomer, and described by him in a letter to that gen-tetnan, which has recently been published in a number of our foreign exchanges, and which will be found of great interest to many of our readers. The following observations are confined to negative paper processes, divided into two— the wet and the dry. The solutions I employ for both these processes are identical and are as follows:— Solution of iodide of potassium, ot the strength of five pirts of iodide to 100 of pure water. Solution of aceto-nitrate of silver in the following proportions ; 15 parts of nitrate of silver ; 20 of glacial acetic acid ; 150 of distilled water. Solution of gallic acid, for developing a saturated solution. Solution of hyposulphite of soda; of the strength of one part of hyposulphite of soda to from six to eight parts of water. The solutions employed are thus reduced to their simplest possible expression, for it will be observed that in iodizing I employ neither rice-water, sugar of milk, fluorine, cyanure, nor free iodide, &c. ; but a simple solution of iodide of potassium (the strength of this solution is a question of considerable importance, not yet, I think, sufficiently investigated) . For both the wet and the dry processes, I iodize my paper as follows. In a tray containing the above solution, I plunge, one by one, as many sheets of paper (twenty, thirty, fifty, &c,,) as are likely to be required for some time. This is done in two or three minutes. I then roll up loosely the whole bundle of sheets, while in the bath; and picking up the roll by the ends, drop it into a cylindrical glass vessel with a foot to it, and pour the solution therein, enough to cover the roll completely (in case it should float up above the surface of the solution, a little piece of glass may be pushed down to rest across the roll of paper, and prevent its rising) . The vessel with the roll of paper is placed under the receiver ot an air-pump, and the air exhausted ; this is accomplished in a very few minutes, and the paper may then be left) five or six minutes in the vacuum.— Should the glass be too high, (the paper being in large sheets) to be inserted under a pneumatic pump-receiver, a stiff lid lined with india-rubber, with a valve in the centre communicating by a tube with a common direct-action air-pump may be employed with equal success. After the paper is thus soaked in vacuo, it is removed and the roll dropped back iato the tray with the solution,and then sheet by sheet picked off and hung up to dry, when as with all other iodized paper, it will keep for an indefinite time. I cannot say that I fully understand the rational of the action of the air-pump, but several valuable advantages are obtained by its use :—1st, the paper is thoroughly iodized, and with an equality throughout, that no amount of soaking procures, for no two sheets of paper are alike, or even one perfect throughout in texture ; and air bulbs are impossible. 2d. The operation is accomplished in a quarter of an hour, which generally employs one, two or more hours. 3rd. To this do I chiefly attribute the fact that my paper is never solarized even iu the brightest sun ; and that it will bear whatever amount of exposure is necessary for the deepest and most impenetrable sha-dows in the view, without injury to the bright lights. WET PROCESS.—To begin with the wet process. Having prepared the above solution of acetonitrate of sliver, float a sheet ot the iodized paper upon the surface of this sensitive bath, leaving it there for about ten minutes. During this interval, having placed the glass or slate of your slider quite level, dip a sheet of thick, clean white printing (unsized) paper, in water, and lay it on the glass or slate as a wet lining to receive the sensitive sheet. An expert manipulator may then, removing the sensitive sheet from the bath, extend it (sensitive side uppermost) on this wet paper lining, without allowing any air globules to intervene. But it is difficult, and a very simple and most effectual mode of avoiding air globules, particularly in handling very large sheets, is as follows :—Pour a thin layer of water (just sufficient not to flow over the sides) upon the lining paper, alter you have extended it on your glass or slate, and then lay down your sensitive paper gently and by degrees, and floating as it were on this layer of water; and when extended, taking the glass and papers between the finger and thumb, by an upper comer, to prevent their slipping, tilt it gently to allow the interposed water to flow off by the bottom, which will leave the two sheets of paper adhering perfectly and closely, without the slightest chance of air-bubbles ; it may then be left for a minute or two, standing upright in the same position, to allow every drop of water to escape ; so that when laid flat again, or placed in the slider, none may turn back and stain the paper. Of course the sensitive side of the sheet is thus left exposed to the uninterrupted action of the lens, no protecting plate of glass being interposed—and even in this dry and warm climate, I find the humidity and the attendant sensitiveness fully preserved for a couple of hours. To develope views thus taken, the ordinary saturated solution of gallic acid is employed, never requiring the addition of nitrate of sil ver; thus preserving the perfect purity and varied modulation of the tints. The fixing is accomplished as usual with hyposulphite of soda, and the negative finally waxed. DRY PROCESS.—In ? reparing sheets for use when dry, for travelling, &c, I have discarded the use of previously waxed paper—thus getting rid of a troublesome operation—and proceed as follows :—Taking a sheet of my iodized paper, instead of floating it, as in the wet process, on the sensitive bath, I plunge it fairly into the bath, where it is lett ta soak for five or six minutes; then, removing it, wash it tor about twenty minutes in a bath, or even two. of distilled water to remove the excess of nitrate of silver, and then hang it up to dry, in lieu of drying it with blotting-paper. Paper thus prepared possesses a greater degree of sensitiveness than waxed paper, and preserves its sensitiveness, not so long as waxed paper, but sufficiently long for all practical purposes, say thirty hours, and even more. The English manufactured paper is far superior for this purpose to the French. To develope these views, a few drops of the solution of nitrate of silver are required in the gallic-acid bath. They are then finally fixed and waxed as usual. These processes appear to me to be reduced to nearly as great a degree of simplicity as possible. I am never troubled with stains or spots, and there is a regularity and certainty in the results that are very satisfactory. You will have observed too, how perfectly the aerial perspective and gradation of tints are preserved ; as also how well the deepest shadows are penetrated and developed— speaking, in fact, as they do to the eye itself in Nature. In exposing for landscape, I throw aside all consideration of the bright lights, and limit the time with reference entirely to the dark and feebly-lighted parts of the view; with a 3 inch lens the time of exDosure has thus varied from ten minutes to an hour and a half, and the action appears to ne never to have ceased. The influence ol the air-pump in this appears to be very sensible, and deserving of further examination and extension. I purpose not only to iodizing, but rendering the paper sensitive with the action of the air-pump, by perhaps suspending the sheet after immersion in the nitrate bath under the receiver of the air pump for a few minutes, before exposure in the camera, or by some other manuvre having the same object in view. I should add, that I have chiefly employed Cansons French paper in iodizing with the aid of the pump. Few of the English manufactured papers are sufficiently tenacious in their sizing to resist the action of the pump, but they may easily be made so ; and were, in short, the English paper, so tar superior in quality to the French, only better sized, that is with glue less easily soluble, even though more impure, there is scarcely any limit to the beauty of the views that might be produced.