have already acted, than on the others, which were, however, gradually changed. Any one conversant with the ordinary photographic processes, must have observed, that when the maximum point of darkening has been arrived at, a peculiar olive tinge gradually comes on*, and the picture being now a fine brown colour on an olive ground may be considered a positive one.

(179.) What is the nature of the change which the iodide of silver undergoes on these Daguerreotype plates? This question has been often asked, and answered in various ways. Draper†, from an experiment in which he placed a paper saturated with starch on an iodized plate, and which, after exposure, gave no evidence of the formation of the iodide of starch, from the liberation of the iodine, infers that no decomposition of the sensitive surface has taken place. Moser contends, that "the action of Light does not necessarily consist in the separation of two chemically combined bodies." He appears to attribute the alteration of colour, to an isomeric change in the iodide of silver.

(180.) It will be found, if we allow one part of a sensitive plate to blacken, whilst the other portion is protected from action, that the dark part is very easily removed by rubbing with the hand, whilst the unchanged division, resists any such mechanical means of removal. In the very fine powder thus rubbed from the silver, I have never been enabled to detect any iodine, but it comports itself, in all respects, like the finely divided metal. A fact mentioned by Moser - viz. that he has been enabled to remove film after film of the sensitive surface, and yet had a surface which was still sensitive to Light, which he considers conclusive in favour of his argument, that the combination of silver and iodine is not

* Moser says, "I exposed iodized plates to daylight, and whenever possible to strong sunlight. The plates became first black, then lighter, and lastly greenish.".

† Philos. Mag., Sept. 1841.

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decomposed appears to me to prove my position, that as the iodide is liberated from the surface, it attacks the layer of silver below it, and, according to the length of time during which the action of the solar rays may be continued, the greater the quantity of silver which is left decomposed on the surface, and the greater the depth of abrasion into the plate. The iodine may attack the plate to a considerably greater depth than we imagine; and hence, although we may remove eight surfaces, we may still have a sensitive surface left. The powerful affinity of this element for the metals, will countenance this opinion, and it will account for the failure of Dr. Draper to produce the iodide of starch in his experiment. It has been shown by M. Lerebour, that the presence of the smallest quantity of iodine vapour in the camera is quite sufficient to prevent the formation of any image; and he distinctly states that he has failed to produce a picture, during the whole of a long summer day, owing to his having kept his iodizing box and camera together, by which the latter received a little of the vapour of this subtle elementary body. Ammonia dissolves off from the plate the darkened portion, whilst a solution of the muriate of soda, or of the hyposulphite of soda, removes the unchanged iodide. These facts certainly support the view, that decomposition has taken place on the sensitive surface, that the iodine has been liberated by the action of the solar rays, and that this change fits parts of the plate for the condensation and combination of the mercurial vapour. There are some facts, however, curiously connected with this action of vapours, which will hereafter claim our attention. Mr. George Shaw has proved that a Daguerreotype plate may be exposed to strong light, and yet immediately restored to its original condition of sensibility by the most transient exposure to the vapour of iodine. See Philosophical Magazine.

(181.) Prismatic Analysis. In the Philosophical Magazine for April, 1840, the author published an ac

count of the effects produced by the spectrum on a Daguerreotype plate. The experiments have been repeatedly tried, and the results have been the same, if we except a little shifting of the point of maximum action. The most refrangible portion of the spectrum appears, after the plate has been exposed to the vapour of mercury, to have impressed its colour; the light and delicate film of mercury which covers that portion, assuming a fine blue tint about the central parts, which are gradually shaded off into a pale grey; and this is again surrounded by a very delicate rose hue, which is lost in a band of a pure white. Beyond this, a protecting influence is powerfully exerted; and, notwithstanding the action of the dispersed light, which is very evident over the plate, a line is left, perfectly free from mercurial deposit, and which, consequently, when viewed by a side light, appears quite dark. The green rays are represented by a line considerably less in size than the luminous green rays. The yellow rays appear to be without action, or to act negatively, the space upon which they fall not receiving the mercurial vapour; and it consequently is seen as a dark band. A white line marks the place of the orange rays. The red rays affect the sensitive surface in a peculiar manner; and we have the inercurial vapour, assuming a molecular arrangement which gives to it a fine rose hue; this tint is surrounded by a line of white vapour, shaded at the lowest extremity with a very soft green. Over the space occupied by the extreme red rays, a protecting influence is again exerted; the space is retained free from mercurial powder. It is not a little remarkable that a protected band is found to surround the whole of the least refrangible rays, and to unite itself with the band which surrounds the rays of greatest refrangibility; it is not equally well defined throughout its whole extent, being most evident from the extreme red to the green; it fades in passing through the blue, and increases again, as it leaves the indigo, until beyond the most refrangible rays it is nearly as strong

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as it is at the calorific end of the spectrum. By lightly rubbing a Daguerreotype image of the prismatic rays, it is obliterated, except over the spaces corresponding with the yellow and red rays. In November, 1842, Dr. Draper forwarded to Sir John Herschel, a specimen of a Daguerreotyped impression of the solar spectrum, obtained by him in the south of Virginia; and in the Philosophical Magazine for February, 1843, Sir John Herschel published an account of his examination of it. In its principal features, this spectrum corresponds with those I have obtained; and I am inclined to attribute the variations which do exist, not merely to a difference in the Light of the sun, but also to some peculiarities in the prisms employed. I am quite inclined, with Dr. Draper, to believe that spectra obtained near to, or at a distance from the equator, will be found to vary considerably. The principal difference which is to be observed between Dr. Draper's spectrum on the Daguerreotype plate, and that which I have described, consists in the remarkable distinctness of the compartments, which are found between the space occupied by the yellow ray and the most refrangible portion of the spectrum. The same protected spaces are observed at the top and bottom of the spectrum; but there is no appearance of the same influence at the sides of the spectral image. Sir John Herschel has shown that the tints observed on this spectral image, are the Newtonian series of colours of the first order of the reflected rings; modified, however, in its first stages, by a cause which seems to have shifted the initial black of that series, to a higher point in the scale of thicknesses of the producing film, or to have displaced the whole series by the intrusion of a white commenceinent. For the Newtonian reflected tints of the first order are black; very feeble and hardly perceptible blue; brilliant white; yellow; orange, at which point the series breaks off. (Herschel.)

(182.) It will not be improper to mention here, that when copper, plated with silver cannot be procured, very

tolerable results may be obtained, by using plates of copper silvered, with the ordinary silvering compound of chloride of silver, common salt, and cream of tartar. The copper being brought to a perfect polish, is well washed with salt and water; a little of the silvering compound is briskly rubbed over the plate, until it presents a uniform coating of white silver; the plate is immediately washed in clean salt and water, and dried near the fire, by carefully rubbing it with very clean cloths. I have found that the polish of this silvering is much improved, by passing the plate through a very weak solution of iodine in water. A thin film of iodide of silver is thus formed. It is then exposed to sunshine, and afterwards rubbed with a clean leather and a little fine prepared chalk, by which the iodized surface is removed, and a beautifully polished face left to operate on. It is then again iodized, and used in the same manner as the plated copper tablets.

(183.) Some very curious results have been obtained by operating on silvered plates, coated with iodine in various ways. A silvered plate, with some leaves placed upon it and pressed close with a glass, was immersed in a solution of iodine in water containing iodide of potassium, and in this state exposed to the sunshine. An iodide of silver was rapidly formed, and blackened; this blackened coating was dissolved off, and another ensued. Thus successive layers of the salt were produced and removed, until at the expiration of an hour, all the silver was gone from the exposed parts of the plate. Upon removing the leaves, it was found that a most beautiful impression of them was made on the copper, and they were of a rich green colour. (184.) A piece of glass was covered with silver leaf, and treated in the same way as the silvered copper. The metal was dissolved from the exposed parts, and a very perfect silver leaf left upon the glass.

(185.) It may be worthy of observation, that the image on the Daguerreotype plates may be completely obliterated by rubbing; but it may be again restored,