THE INFLUENCE OF ATMOSPHERIC CHANGES. 365

"Having made my experiments with the greatest care, I seize with satisfaction this manner of explaining phenomena apparently contradictory. It would, indeed, be curious and interesting to find that neither myself nor the other experimenters are in error, and that we differ only as regards conditions and circumstances, which, without our knowledge, have exercised an influence upon our experiments. But there can be no doubt as regards the iodised plate, when it has been subsequently submitted to the vapour of bromine alone, or of bromine and chlorine united. I have operated with these substances during the various periods of the year, and I have invariably found that the red, orange, and yellow glasses destroy the action of daylight."

(626.) On the influence of atmospheric changes the following remarks are of importance:—

"The possible simultaneous existence of two antago nistic lights reflected from the atmosphere explains those anomalies which are so annoying to photographers; for example, those circumstances under which it seems impossible to obtain any image, whatever may be the length of exposure in the camera. in the camera. This difficulty is imputed to the preparation of the plates, the state of the accelerating solutions, &c.; and after all it only exists in the light. As much effect is often obtained in 30 seconds as in 120 seconds, for it is possible to have obtained a first effect which has been afterwards destroyed; and that must inevitably be the case if, during the latter part of the exposure in the camera obscura, any clouds or vapours have suddenly rendered the light of the sun yellow; in this case the object which had first reflected white light, becoming capable of reflecting only yellow light, this last must destroy the effect produced by the first.

"If the object reflects at the same time blue light from the zenith and yellow light from the southern horizon, the two lights may be neutralised and destroy each other, so that no photogenic effect can be obtained. Sometimes,

when the sky is cloudless, the space at the zenith is of a particularly deep blue colour; whereas the vapours produced by the heat of the sun make the sky appear slightly tinted with yellow, from the horizon to a certain height. In those circumstances all photographers must have observed that the operation in the camera obscura is excessively slow.

(627.) "This may probably explain those dark specimens, which were so weak in effect, obtained some years ago on the Alps during clear and cloudless weather, by an operator sent to Italy by M. Lerebours to take Daguerréotype views. M. Lerebours' operator was an experienced photographer, and he found, to his great surprise, that on the summit of the Alps he could not obtain any effect in less than five or six times what he considered to be the necessary exposure. He was surrounded with snow, the sun was shining in all its brilliancy, the sky was pure and cloudless, of a deep blue colour, whereas the horizon was without doubt slightly tinted yellow, on account of the vapours produced by the melting of the snow and ice of the glaciers.

"Mr. Lerebours' operator was using at that time (in 1840) plates only iodised; and this fact would confirm the neutralisation of the photogenic effect upon plates of iodide of silver, by the less refrangible rays acting simultaneously with white light, according to Dr. Draper's observations.

"It would be interesting if M. Lerebours could state the period of the year in which his artist operated on the Alps; although it might be that from the high elevation of those regions the operator was precisely in those atmospheric conditions which exist on less elevated grounds during only certain months, on account of the density of the atmosphere that the sun's rays have to penetrate."

(628.) When M. Arago made his "Report on the Daguerréotype," before the Chamber of Deputies in 1839, he used these words:-" Upon examining several of the pictures to be submitted for your inspection, all will con

VARIATIONS DEPENDENT ON LATITUDE.

367

sider the immense advantages which would have been derived, during the expedition to Egypt for example, of a means of reproduction, so exact and so rapid; all will be struck with this reflection that, if photography had been known in 1798, we should this day have possessed faithful representations of many valuable antiquities now, through the cupidity of the Arabs and the vandalism of certain travellers, lost for ever to the learned world." Now, this hope was long before it was entirely fulfilled. It was of course imagined that, under the brilliant sun and clear skies of the south, photographic pictures would be produced with much greater quickness than they could be in the atmosphere of Paris. It was found, however, that with the original Daguerréotype plate a much longer time was required. Even in the clear and beautiful light of the higher Alps, it has been proved that the production of a photographic picture requires many minutes more, even with the most sensitive preparations, than it does in London. It has also been found that, under the influence of the brilliant light of Mexico, twenty minutes, and half an hour, were required to produce effects, which in England would occupy but a minute; and travellers engaged in copying the antiquities of Yutacan have on several occasions abandoned the use of the photographic camera, and taken to their sketch-books. Dr. Draper has observed a similar difference between the chemical action of the Light in New York and in Virginia. This can only be explained on the supposition that the intensity of the Light and Heat of those climes interferes with the action of the actinic rays on the sensitive preparations which were employed. Dr. Draper furnished a prismatic image, impressed upon a Daguerréotype plate in Virginia, which exhibited many remarkable peculiarities: it has been described in a previous chapter. Nothing could be more valuable than a series of such pictures of the spectrum, produced in different degrees of latitude, and at different elevations. It would be very easy to procure

them from our magnetic observatories in different parts of the world, and these would afford much valuable information on this point. Where an organic compound is combined with the silver salt employed, the sensibility is so quickened that chemical changes take place under the most intense Light and Heat of tropical climes.

(629.) These numerous facts group themselves together thus:

LIGHT is white, colourless, until it comes in contact with matter. It then suffers variable degrees of decomposition, by reflection, refraction, dispersion, and unequal absorption, and the result is colour. Rays of a particular colour are found to occupy, in a marked manner, particular divisions of the spectrum. Red occupying the lower edge, or the least refracted portion, while rays of the blue class occupy the other extremity, or the limits of greatest refraction.

HEAT, in its radiant state, produces no sensation of colour, it occasions the separation of the particles of matter from each other, and by the encreasing bulk of the mercury in a thermometer tube, or by the drying of a volatile fluid, we determine the fact, that LIGHT and HEAT do not obey the same laws of refraction. The maximum of HEAT being found below the red rays, that of LIGHT, considerably above them in the yellow rays.

ACTINISM, produces none of the phenomena of Light or Heat. It is much more refrangible than either. Where Light and Heat exist in their greatest force actinism cannot exert its power; therefore, on all the preparations of gold, silver, &c., the spaces covered by the yellow and the red rays produce no effect, but the maximum of chemical change is found to exist where there is least | Light, and scarcely a trace of Heat.

(630.) It has been stated that every ray has a definite amount of chemical action, and that by obstructing that ray we obstruct the chemical action, that they are in fact inseparable. Reference to the experiments detailed will

RELATIONS OF LIGHT AND ACTINISM.

369

show that such is not the fact. The blue ray, to quote one example, has freely passed a coloured transparent medium, and yet been entirely deprived of chemical power. It has been obliterated and the chemical power has remained unchanged.

(631.) Again, the peculiar fluorescent phenomenon of the spectrum has been spoken of as the chemical rays rendered luminous-made visible; yet, by a block of uranium glass we stop back the fluorescent rays, and allow the permeation of rays which are chemically active. Beyond this, it appears, that we have the power of developing chemical action in the yellow rays, where it was previously masked, by the use of the same medium.

(632.) LIGHT, in its pure condition, as light, capable of being decomposed into the primary rays, will not produce chemical change.

The great exceptions to this are:-organic compounds, which, it has been shown, will change under the influence of Light, separated as far as we possibly can separate it from the chemical principle,-and vital organisms, which appear to require the peculiar excitement due to the luminous principle of the solar rays for the support of that great living principle to which growth belongs. No chemical compound of any of the inorganic elements has yet been discovered upon which pure Light produces a che

mical change.

HEAT, as a radiant force, in some cases does effect a change, but it is of an essentially different character from that produced by actinism. On these grounds, therefore, I still hold to my belief, that the chemical power of the sunbeam, ACTINISM, is a physical agent widely different from LIGHT or HEAT, but associated with these principles in their modes of motion.

BB