with a prismatic analysis from Sir John Herschel, which quite confirms my own results,-the spectrum this distinguished philosopher obtained may be represented by the following curve — the maximum and minimum, or rather the positive and negative, effect being shown.

(145.) Advantage has been taken of this bleaching property for the production of positive photographs by the first process. The plan pursued is, essentially, to darken the paper after the application of the ferrocyanate, and then to throw the balance in favour of the whitening effect, by washing over the paper with a tolerably strong solution of the iodide of potassium. Some processes, which are much more certain, and which give more permanent pictures, will presently be described.

(146.) The increased sensibility of this paper appears to depend on the united decomposition of the ferrocyanide of potassium and the iodide of silver. It is well known that the ferrocyanide of potassium is decomposed by prolonged exposure to the sun's rays, and a portion of prussian blue formed. It will be found that this decomposition is brought about with much greater rapidity, if iodide of potassium is mixed with the ferrocyanide; and this appears to be the case with most of the metallic iodides: with the iodide of silver this is very decided. Chemical philosophy affords us numerous examples, of the power which one body possesses of setting up an action similar to its own, in bodies brought into contact with it. Here we have two bodies in, what appears to be, almost chemical union, each one subject to change under solar power, exerting this catalytic influence, as it has been called, upon each other.

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(147.) The ferrocyanide of potassium will, in all probability, be found equally useful in quickening the change upon other photographic materials. I find that it accelerates the darkening action on the bromide of silver considerably; and it acts powerfully upon the chloride under some conditions.

(148.) Positive Photographs formed by the Salts of Iodine. - If photographic paper, which has been darkened by exposure, is washed over with the iodide of potassium, it is speedily whitened by the solar rays upon subsequent exposure. The interesting character of the photographic processes founded upon this peculiarity led me to pay considerable attention to them. The results of my inquiries were published at the time in the "Philosophical Magazine,"* since which nothing has been added to our knowledge of the subject.

(149.) To avoid unnecessarily complicating this part of my treatise, I shall refrain from entering into that very minute detail which is given in the paper above referred to, and confine myself to an explanation of the best mode of preparing a good photographic paper, on which perfect pictures may be produced in the camera or otherwise, having their lights and shadows correct as in nature by one operation. It is necessary to remark, that more exact attention is required in the preparation of this kind of paper than in almost any other; very trifling differences in the proportions of the ingredients used, and in the time occupied in the first darkening process, completely changing the result, as will be hereafter explained.

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(150.) In preparing this kind of paper, almost any the sensitive salts of silver may be used. Dr. Fyfe prefers the phosphate of silver. Lassaigne, Bayard, and Talbot use the chloride of silver. Verignon, however, recommends a combination of the chloride and bromide of silver.

* Philosophical Magazine, Vol. xvii. No. 109., September, 1840.

My own experience is much in favour of the chloride. Hydro-chloride of ammonia and chloride of barium produce much better effects than any other salts in the production of the chloride of silver, the bleaching action being more easily excited, and the resulting pictures being of a more beautiful character.

Good letter paper is soaked for five or ten minutes in solutions of either of the above salts-forty grains of the salt to four ounces of water. Each sheet is then carefully removed, and being laid on a porcelain or marble slab, gently wiped over with very clean linen, and then dried. When dry, the paper being pinned out upon a board, it is washed over with the following solution:

Take of crystallised nitrate of silver 120 grains, and dissolve it in twelve fluid drachms of distilled water; to this solution add four fluid drachms of alcohol, which will render the mixture opake. After a few hours, a minute quantity of a dark precipitate falls, which must be separated by the filter.

This solution must be applied with a very soft sponge brush, boldly, but lightly, over one surface, and the paper carried directly into the sunshine. It is instantly changed, but usually it darkens at first very unequally, owing to the irregular absorption of the fluid by the paper. After it has been exposed for a few minutes, it is removed, and being again washed over with the argentine solution, it is a second time exposed, and kept in the sunshine, until a very fine chocolate brown colour is uniformly produced. It is then dried in the dark and preserved for use: it seldom happens that the paper is exposed sufficiently long to dry it perfectly.

(151.) It is necessary that great attention should be paid to the character of the Light to which, in this part of the process, these papers are submitted. The morning sun should be chosen, and a perfectly cloudless sky, if possible. It may appear unlikely, but nothing is really more true,

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than, that these papers indicate to the practised eye, during the bleaching operation, the effects of every cloud which has obscured the sun's disc during the darkening process. A peculiar film, as if the washes had been applied with a dirty brush, is produced by every such check.

(152.) To use the papers thus prepared, it is required that they should be washed over with some iodide, and exposed to the sun's influence wet. The iodide of potassium, being the salt which is most easily obtained, will be generally preferred. It is very difficult to decide on the best proportions in which this salt should be used, the difference of a few grains only, wonderfully altering the result; in general, about 30 grains of a pure salt to one ounce of water, will be found to produce the best effect. In some experiments instituted to settle this important point, it was found that papers washed with a solution containing 100 grains to the ounce, required twelve minutes. to bleach in the direct rays of the sun; whereas, papers washed in a solution of the strength above recommended, took but four minutes. For the camera obscura I would recommend the use of the iodide of barium, beyond every other preparation; and if by throwing down some of the baryta by a drop or two of dilute sulphuric acid, we set free a little hydriodic acid, it acts much more energetically on the darkened paper; giving in the camera, provided a good clear image is formed, a very beautiful positive picture in less than half an hour. When engravings are required to be copied, which they may be most beautifully by this process, they should be soaked in water, and superimposed on the photographic paper quite wet. The object of this is two-fold-to insure transparency, and the closest possible contact, the interposition of even films of air interfering with the result. Although it may appear, that there is much which is perplexing in this process, a little attention will soon render any one perfect in the manipulatory details, and then the results are certain. Pictures taken from nature with the camera in this manner, possess ex

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treme beauty. The fine contrast of the shadows with the lights, give them the character of finished sæpia drawings; and the gradations of tint, corresponding with the amount of Light radiated from different objects, are very pleasing.

(153.) These drawings may be most perfectly fixed, provided they are kept in a portfolio, and only exposed to the sunshine occasionally, by washing them in clean water only, which removes all the iodide that has not been decomposed. If, however, the drawings so fixed are exposed continually to Light and to the influence of atmospheric changes, they slowly fade out, and in a month or two no trace of a picture remains. This may may be thus explained :— If a darkened paper is washed over with an iodide and exposed to sunshine, it is at first bleached, becoming yellow; then, if long exposed, it again darkens. If in this state it is put aside in the dark, it will in a few days be completely bleached; by exposure to sunshine it may be again darkened, but not so readily as at first, and the yellow colour is again restored in the dark.

(154.) If a darkened paper, bleached by an iodide and sunshine, again darkened, and then placed in a bottle of water, the yellow colour is much more quickly restored, and bubbles of gas will escape freely, which will be found on examination to be oxygen. If placed in an exhausted vessel and hermetically sealed, the drawings thus formed are quite permanent, however much they may be exposed to sunshine. From this it is evident that the gradual fading arises from the influence of atmospheric moisture. The water is slowly decomposed under luminous influence, the hydrogen unites with the iodine to form hydriodic acid, which converts the darkened portion) which is finely divided silver) again into the yellow iodide of silver.

(155.) A few interesting experiments will place this question in a much clearer light. Precipitate with any iodine salt, iodide of silver from a solution of the nitrate, and expose the vessel containing it, liquid and all, to sun