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would prove, not only that the spectrum formed by these invisible rays is continuous, that there are solutions of continuity as in the visible spectrum, but also that in the two superposed spectra these solutions exactly correspond.”

(607.) Reference back to the chapter, which treats of the influence of absorptive media upon the chemical action of the solar rays, will be necessary in the consideration of the connection between Light and actinism. The following paragraphs, to (603.), are preserved as they were written in 1844.

(608.) If we pass the sunbeams through a deep yellow glass, or, which is still better, a solution of the sulphate of chromium, we rob them of but very little of their Light or Heat; but it will be found that they are deprived of the greatest part of their chemical influence. Sensitive papers may be exposed behind such a glass or such a fluid for a considerable length of time without undergoing any change.

(609.) An instance of the intense power of pale yellow media is the following :-Dalton's solution of quadrosulphuret of lime, prepared by boiling lime and sulphur in plenty of water, scarcely seems to impair the brightness of white objects seen through it in thicknesses of an inch or two.

The spectral image, passed through such a fluid, exhibited but small apparent loss of illuminative power; but its total photographic effect must have been diminished, by the loss of at least four-fifths of its amount.” (Herschel.)

(610.) A coloured atınosphere acts in the same manner. The case mentioned by Sir John Herschel in his Memoir “ On the Chemical Action of the Rays of the Solar Spectrum,” which I will quote, is to the point:-“ It is probable that other atmospheric relations than those which refer to the extinction of the merely luminous rays are concerned in this phenomenon. The tint of coal smoke is yellow (as may be seen in perfection in a London November fog), and more than one instance of the intense 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 common 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 VARIATIONS IN ACTINIC POWER.


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 inentions 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

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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,-.

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