power and capricious singularities of very pale yellow media in their action on the chemical rays will come hereafter under our notice. In the locality from which this paper is dated (Slough), a light easterly wind brings with it abundant smoky haze from London, to which rural prejudices assign the name of 'blight,' and attribute an insect origin. On such occasions, when the sky has been otherwise cloudless, I have been continually at once annoyed and surprised by the slowness of photographic action, and by the fugitive nature of its results under the process of fixing."

(611.) The power which chlorine gas, diluted with commcn air, has of absorbing this chemical principle is very great. Although so pale a yellow is the mixture, it does not appear that there is any actual loss of Light, by the interposition of a vessel thus filled, over that which would occur with a bottle of common air: it will be found that the influence producing chemical change is absorbed in a remarkable manner, and photographic preparations of the most sensitive kinds, change with the utmost slowness, behind even this pale yellow gaseous medium.

(612.) At the Cork meeting of the British Association a paper from Dr. Draper was read, detailing the following experiment, which is interesting :-Within a very large vessel, filled with a mixture of chlorine and hydrogen, was placed a small one filled with the same gaseous mixture; and this arrangement was exposed to the sunshine. Of course the gases in the outer vessel speedily combined, and the vessel was filled with the vapour of muriatic acid; but the gases in the interior vessel, although Dr. Draper has shown that an exceedingly small influence will occasion them to combine, were found to be unaffected. The atmosphere surrounding this inner vessel had, although allowing the passage of a flood of Light, separated the chemical principle, which alone had the power of inducing the combination.

(613.) It has been observed by Daguerre, and almost by every photographist since the announcement of his

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discovery, that the sun two hours after it has passed the meridian, is much less effective in the photographic processes, than it is two hours previously to its having reached that point. May not this depend upon an absorptive power of the air, which we may reasonably suppose to be more charged with vapour two hours after, than two hours before, noon?

(614.) If we take a considerable thickness of a dense purple fluid, as, for instance, a solution of the ammoniasulphate of copper, we shall find that the quantity of Light is considerably diminished — at least four-fifths of the luminous rays are absorbed; but the so-called "chemical rays" permeate it with the greatest facility, and sensitive preparations are affected by this influence, notwithstanding the deficiency of Light, nearly as powerfully as if exposed to the undecomposed sunbeams. Those deep blue glasses which are coloured with cobalt, and which are commonly used for finger glasses, have nearly the same effect of obstructing LIGHT, but allowing the free passage of this principle which accompanies it.

(615.) In the valuable paper to which I have already several times referred, Sir John Herschel mentions the curious property of the muriate of chromium, "which reduces the spectrum to two narrow and pretty well defined spaces, coloured the one red and the other green, the red being that of the extremity of the spectrum, and the green of great purity and richness of tint." On photographic papers this analysed spectrum impresses two circular spots, whose centres coincide with those coloured images exactly in the green, and nearly so in the red: one spot is intensely black, and the other white. The same authority has also proved that a preparation of the colouring matter archil-the rocellate of potash-admits the permeation of a great quantity of green light; "such as, had its properties not been altered by the medium it had traversed, could not have failed in the time the exposure lasted to have produced a considerable blackening

of the paper," which was however unchanged over the green luminous spaces. These experiments are very illustrative: they show the existence of a chemical principle in the region, but at the same time quite independent of the colour of the ray.

(616.) If we examine the photographic images impressed by the spectrum itself, as represented in the plate, or any other series, it will be seen that the luminous rays

occupy but a very small space compared with the influences of heat and chemical power. If the linear measure, or the diameter of a circle which shall include the luminous rays, is 25, that of the calorific spectrum will be 42.10, and of the chemical spectrum 55 10. Such a series of circles may well be used to represent a beam from the sun, which may be regarded as an atom of Light surrounded with an invisible atmosphere of Heat, and another still more extended, which possesses the remarkable property of producing chemical and molecular change.

(617.) We may regard the centre or maximum of this

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power as situated somewhere about the most refrangible blue or the lower edge of the indigo ray, the shifting of this point, as we vary the materials, preventing us from fixing the spot with precision. Towards the most refrangible end of the spectrum we find, as might be expected, the power slowly diminishing in force with the most perfect regularity, until at a certain point, even the most sensitive preparations can detect no chemical action. On the contrary, as we approach the most luminous rays, the action is abruptly stopped, the light-giving power has interfered with the chemical power, and in a great many cases all action ceases at this point. In some others this point, where the effect of Light is the greatest, having been past, the chemical power is again exerted, and a similar interference is, in some cases, strikingly shown by the maximum calorific rays, although we have abundant evidence of a chemical principle extending far into that invisible region of the spectrum.

(618.) The action of the red rays of the prismatic spectrum, which it is now important we should consider, has invited the examination of a great number of experimentalists; and some varied, and apparently opposing, results appear to have been obtained. In 1839 Sir J. Herschel first pointed out that the least refrangible rays of the spectrum exerted a protecting action upon several photographic preparations. Subsequently M. Edmond Becquerel was led to believe that the red, orange, and yellow rays had the power of continuing the action which had been commenced by the more refrangible rays. He therefore divided the rays of the spectrum into two sets, giving to the rays existing at the blue end of the spectrum the name of exciting rays and to those at the red end the name of continuing rays. M. Gaudin, shortly after the publication of Becquerel's Memoir, stated that, having placed a Daguerréotype plate in the camera obscura, and allowed the lenticular image to act upon it for a time quite insufficient for the development of any visible image,—.

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 rays fall is very rapidly blackened.

red

(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