he placed it beneath a yellow glass.-he succeeded in producing a complete development of the image without the application of mercurial vapour.

(619.) Upon the Daguerréotype plate and upon the ordinary photographic papers a dissimilar action is constantly exerted by the rays at the two extreme ends of the spectrum. On the light-coloured salts of silver on paper, the rays above the green darken, while for a considerable time those below the green protect the paper from change. That is, they not merely do not blacken themselves, but they retard, and in many cases entirely check, the action of rays which do produce that chemical change which is indicated by darkening the white surface. The effects upon the Daguerréotype plate are similar to those. It has been stated that if the papers or plates are affected by the chemical radiations of the blue end of the spectrum, and then brought under the influence of the red end, that the action. is continued, as Becquerel's and Gaudin's Experiments appear to show. This is not constantly the case, indeed it is doubtful if the actions are the same in any instance. Mr. Wilson, in 1776, stated that the most refrangible rays excited phosphorescence in the sulphuret of lime, but the less refrangible ones extinguish it when shining. Ritter found that chloride of silver, darkened in the violet rays, had its colour partially restored by the red rays. Wollaston showed a similar result in the action of the spectrum on guaiacum; and this has been confirmed by Sir John Herschel. I have shown, that if we wash a paper covered with darkened chloride of silver with a solution of iodide of potassium, and expose it to the action of the spectrum, that portion covered by the blue, violet, and extra-spectral rays is whitened, while that upon which the yellow and red rays fall is very rapidly blackened.

(620.) The Daguerréotype picture is, as is well known, developed by the agency of mercury (167.). The iodide, bromo-iodide, or whatever preparation of silver may be employed, is decomposed by the action of the chemical

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radiations, and the mercury attacks the portions of the plate over which this change has taken place, and always in proportion to the amount of change. The moment chemical change commences, the dormant picture is formed, and it has been shown by M. Claudet that the effect is almost instantaneous. As iodine is liberated it attacks the surface of the plate immediately beneath the film which has first undergone a change; and this is continued, until eventually the plate is blackened, or solarised, as the technical expression is. As in the case of the chloride on paper darkened by exposure to the most refrangible rays, which loses colour under the action of the red rays, so a darkened Daguerréotype is considerably lightened by exposure under a mono chromatic red glass. Now, M. Gaudin's image does not appear to be produced by a class of rays which can strictly bear the name of the continuing rays. The action by which an image yet dormant on the silver plate is evoked by the rays which belong particularly to the Heat and Light spectrum, appears to belong to the physical phenomena described under the general head of thermography. The iodide of silver has suffered that disturbance by which the chemical affinities are loosened. Then, by covering the plate with a yellow or a red medium, more especially the latter, the Heat or Light rays-sometimes both-separate the particles more directly from each other, the iodine is no longer held by surface (epipolic) force, and from the arrangement of the finely divided silver the picture results. These examples, and several others which have been noticed, all lead to the conviction that these peculiarities are merely the balance of action between the three forces of the sunbeam— LIGHT, HEAT, and ACTINISM.

(621.) As it regards the continuing action of M. Becquerel, we find that a prolonged exposure of the plate to the spectrum does not produce the result. Dr. Draper first observed this, and I have repeatedly verified his results: "I found that it was perfectly immaterial

whether the exposure to the spectrum was for thirty seconds or one hour, the result was the same. The final action had been produced, the less refrangible rays had carried their region (the part of the plate on which they fell) to the unaffected state, while the more refrangible. had solarised theirs. Now, if the phenomenon was due, as M. E. Becquerel supposes, to an unequal action of the same kind in different rays, it is obvious that the final result ought to depend on the time of exposure; the red ray, aided by daylight, should carry its portion through the various shades of white, and solarise it at last. But this, in the longest exposure, never takes place; that part of the plate remains as though a ray of light had never fallen upon it."

(622.) We have some curious examples of the protecting actions alluded to. M. Claudet endeavours to take an impression of the sun, when seen as a red globe through a mist. The Daguerréotype plate is sensibly affected by the diffused daylight over every part, but where the sun's image falls there is not a trace of an image. The solar rays in this example protect the plate from the influence of the diffused daylight which is sufficiently active to produce a chemical change over every other part of the sensitive surface.

(623.) Herschel noticed an extended action of this kind on the tartrate of silver, and about the same time I observed it on a Daguerréotype plate. In these examples the protected space was extended around the spectrum, and observable even above the violet. Some experiments and remarks by M. Claudet and Dr. Draper are of much importance. Mr. Claudet was puzzled with some apparent contradictions in the results obtained; at one time he finds red and yellow Light continuing the action, and at another time destroying it. M. Claudet then remarks:

(624.) "I have again examined my former experiments, and I find some specimens which show that red and yellow glasses have destroyed the effect of the photogenic Light on plates simply iodised. How can this fact be reconciled

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with Messrs. Ed. Becquerel and Gaudin's experiments, and those I had just made in consequence of these philosophers' observations? Had I made any mistake in the classification of my former specimens, or had I erred in my mode of operating? This might be, and I feared such was the case, as I just obtained quite different results. I then questioned my assistant, and he recollected well that we had repeatedly experimented upon plates simply iodised, and that we had then found that red and yellow glasses did destroy the action of daylight, as well on iodised plates as on those which had been submitted to the compound vapour of iodine and bromine.

"I then recollected a curious fact mentioned by Dr. Draper of New York, which at the time of its publication I had found so inexplicable that I did not pay much attention to it, and which I had totally forgotten during the course of my experiments.

"Dr. Draper said:-- Such are the facts I observed, and they seem to have been reproduced by MM. Foucault and Fizeau; but there are also others of a much more singular nature. In these Virginia specimens the same protecting action reappears beyond the violet.

"The only impressions in which I have ever seen this protecting action beyond the violet, are those made in Virginia in 1842; they were made in the month of July. Struck with this peculiarity, on my return to New York the following August I made many attempts to obtain similar specimens, but in no instance could the extra-violet protecting action be traced, though the analogous action of the red, orange, yellow, green and blue, was perfectly given. Supposing, therefore, that the difference must be due either to impurities in the iodine or to differences in the method of conducting the experiment, I tried it again and again in every possible way. To my surprise I soon found that the negative effect was gradually disappearing; and on Sept. 29. it could no longer be traced, except at the highest part corresponding to the yellow and green rays. In December it had become still more imperfect, but on

the 19th of the following March the red and orange rays had recovered their original protective power. It seemed, therefore, that in the early part of the year a protective action had made its appearance in the red ray, and about July extended over all the less refrangible regions, and as the year went on it had retreated upwards.

"Are there, then, periodic changes in the nature of the sun's light?' &c.

(625.) "From these experiments of Dr. Draper it would appear that, according to the months of the year in which we operate, the red and yellow rays either do or do not exercise a destructive action. It must be remarked that Dr. Draper mentions only iodised plates, that he always speaks of iodine alone, and that he never alludes to bromine or chlorine, which were hardly in use at the time of his experiments.

"Dr. Draper adds:-'I further found, that when different rays are brought to act upon each other, the result does not alone depend upon their intrinsic differences, but also on their relative intensities. Thus the green and lower half of the blue rays, when of a certain intensity, protect the plate from the action of the daylight; but if of a less intensity, they aid the daylight.

"The red and orange rays, when of a certain intensity, increase the action of daylight on the plate; but if of a less intensity, they restrain it.'

"It would result from this last observation of Dr. Draper, that when the red and orange rays are not endowed with the destructive action, they, on the contrary, have the property of continuing or assisting the action of day light.

"Is it not, then, possible, that, like Dr. Draper, I may have made my first experiments on the iodised plate during the period when the red and yellow rays were endowed with their destructive action, and that Messrs. Becquerel and Gaudin may have made theirs when these rays had lost their destructive, and had acquired their continuing action, at a period corresponding to the present?