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however, appears to imply the contrary, in a note which he has appended to M. Niepce's paper. This heliographic process was exceedingly tedious and uncertain. An exposure of two or three hours was necessary to produce an impression from an engraving, even under the influence of a bright sun; and in the camera obscura, the plate was left under the influence of strong Light for six or eight hours, and sometimes even longer, before a tolerable picture could be produced. M. M. Daguerre materially modified and improved this process. The resin of the essential oil of lavender, dissolved in alcohol, was found by him to be more susceptible of change than the bitumen; and instead of washing the plate with the solvent recom. mended by M. Niepce, which often removed all the varnish from it, he exposed the tablet to the vapour of petroleum, by which a much more certain effect was produced.

(36.) A correspondence between M. Niepce and M. Daguerre has been published, which sufficiently proves, that to the latter the sole merit of the discovery of the process which bears his name is due. In 1831-2, M. Niepce indeed regrets that, at the recommendation of M. Daguerre, he had lost so much time in experiments on iodine. "I repeat it, Sir," he says, "I do not see that we can hope to derive any advantage from this process, more than from any other method which depends upon the use of metallic oxides," &c. &c. In another letter he speaks of a decoction of thlapsi* (shepherd's purse), fumes of phosphorus, and particularly of sulphur, as acting on silver in the same way as iodine, and that caloric produced the same effect by oxidising the metal, "for from this cause proceeded in all these instances this extreme sensibility to light." We may perceive, from these remarks, that although M. Niepce may not have been fortunate enough to discover the exquisitely sensitive method of M. Daguerre, he must have submitted to experiment a great variety of substances

* Thlapsi Bursa-pastoris (Linn.), Capsélla Bursa-P. (De Candole.)

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in different states of combination. The philosopher of Châlons died in July, 1833, and a new agreement was entered into between his son, M. Isidore Niepce, and Daguerre.

(37.) In January, 1839, the discovery of M. Daguerre was reported, and specimens shown to the scientific world of Paris. The extreme fidelity, the beautiful gradations of light and shadow, the minuteness, and the extraordinary character of these pictured tablets, took all by surprise. Europe and the New World were alike astonished at the fact, that Light could be made to delineate on solid bodies, delicately beautiful pictures, geometrically true, of those objects which it illuminated. In the July following, after a bill was passed, securing to M. Daguerre a pension for life of 6000 francs, and to M. Isidore Niepce of 4000 francs, with one half in reversion to their widows, the process by which these pictures were produced was published.

(38.) Mr. Henry Fox Talbot informs us that in 1834 he began some experiments with a view of rendering the images of the camera obscura permanent. On the 31st of January, 1839, six months prior to the publication of M. Daguerre's process, a paper giving an account of Mr. Talbot's labours, entitled, "Some Account of the Art of Photogenic Drawing, or the Process by which Natural Objects may be made to delineate themselves without the aid of the Artist's Pencil," was read before the Royal Society; and in another communication on the 21st of February, 1839, the method of preparing the paper was given, and the process by which the design was fixed described. In these, however, there will be found but little advance upon the process of Mr. Wedgwood: the mode of fixing, which was the only novelty proposed by Mr. Talbot, was imperfect; the process of fixing by the use of the hyposulphite of soda belonging to Sir J. Herschel and Daguerre.

(39.) It will be evident to all, that the researches of the French artist and of the English philosopher were pursued,

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without any knowledge of each other's labours. The results in both cases were satisfactory, and they equally rendered most important service; to Science, in producing an instrument by which some of the mysterious phenomena of Light could be investigated, and to Art, by giving its votaries tablets, upon which Nature impresses herself in all her delicacy and decision, in all her softness and her grandeur, and in all her richness of tone and breadth of effect. Colour alone is wanting, and there are sufficient reasons for believing, that in the progress of research, we shall, before long, arrive at processes, by which the delightful pictures of the camera obscura shall be rendered permanent in all the beauty of those glowing tints, which give to the fields of creation their exquisite charm and enchanting character.

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CHAPTER II.

ANALYSIS OF THE SUNBEAM.

(40.) It is important on entering on the study of a set of phenomena, which are still among the novelties of science, that we should make ourselves acquainted with all the conditions, which may directly or indirectly influence them. The chemical changes produced by the solar rays have usually been regarded as a function of Light (the luminous principle as distinguished from heat or any other power). It has been stated in the historical chapter (30) that some of the Continental philosophers, and particularly Berard, were disposed to refer the phenomena of chemical change under the influence of radiant action, to some principle which, although associated with Light in the sunbeam, as heat is, was still to be distinguished from Light, by its being incapable of producing visibility, or colour. The object of this volume being the full examination of this problem, it is proposed to consider the phenomena of the strictly luminous radiations, and of the heat radiations, previously to entering on the investigation of the chemistry of the sunbeam.

(41.) If a hole is made in the window-shutter of a darkened room, there will be, if the sun is shining, a bright circular image formed upon the floor, or on a screen placed to receive it. This circular spot of Light is an image of the sun, as we may prove by placing the eye, protected by a smoked glass, in the path of the sunbeam, when that luminary will be distinctly seen through the hole, or, if we receive such an image when the sun is partially eclipsed, it will exhibit only the still luminous portion of the disc.

(42.) If in the path of the beam we interpose a prism

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of glass, as A B C in the accompanying figure, having one of its angles downwards, and so adjusted to the incident sunbeam, that it falls obliquely on one of its sides, a c, the pencil of light will be turned out of its path or refracted. The luminous image will now be thrown upwards, and, instead of forming a circular white spot, it will be elongated into a flame like image, consisting of several beautifully coloured bands, R V. This image is called a spectrum; we speak of it as a solar or a prismatic spectrum, and it is sometimes termed the Newtonian spectrum, since the first satisfactory examination of the solar beam by prismatic decomposition was performed by Sir Isaac Newton.

(43.) Sir Isaac Newton, in 1666, published the results of his researches. He determined the number of rays, into which white Light was decomposed by the prism, as seven, and that the respective length of each ray was relatively as indicated in the following table. I have placed by the side of Newton's measurements the results of Frannhofer's more recent examination, with far more perfect instruments than any which Newton could command. The differences are in many respects striking, but since under any circumstances these measurements must depend upon the power of the eye of the observer to distinguish accurately the line between two colours fading one into the other, it is scarcely possible that any two observers should give the same limits to any particular coloured ray. I have frequently tried the experiment of desiring different people to mark with a needle the commencement and termination