by the Agency of Light. 467 sensitive tablet, and with indelible lines their precise forms, and the lights and shadows by which these forms are modified. He thus gives permanency to details which the eye itself is too dull to appreciate, and he represents Nature as she is-neither pruned by his taste, nor decked by his imagination. From among the countless images of surrounding objects which are actually accumulated in every part of space, he excludes, by means of his darkened chamber, all but the one he wishes to perpetuate, and he can thus exhibit and fix in succession all those floating images and subtile forms which Epicurus fancied, and Lucretius sung. * The art of photography, or that of delineating objects by the agency of the light which they radiate or reflect, is substantially a new invention, which we owe to two individuals, Mr. Talbot and M. Daguerre, although, like all other arts, some approxima tion had been made to it by previous inquirers. So early as 1802, Mr. Thomas Wedgewood, the celebrated porcelain manufacturer, published in the Journals of the Royal Institution, A method of copying paintings upon glass, and of making profiles by the agency of light upon nitrate of silver, which was accompanied with some observations by Sir Humphry Davy. Having ascer tained "that white paper or white leather, moistened with a solution of nitrate of silver, undergoes no change when kept in a dark place," but "speedily changes colour" when "exposed to the daylight," Mr. Wedgewood found "that the alterations of colour took place more speedily in proportion as the light was more intense;" that the full effect was produced by the sun's light in two or three minutes, whereas two or three hours were required in the shade; that the red rays have little action upon it, the yellow and green more, and the blue and violet most of all. "Hence," says Mr. Wedgewood, "when a white surface covered with a solution of nitrate of silver, is placed behind a painting on glass exposed to the solar light, the rays transmitted *Dico igitur, rerum effigias, tenuisque figuras Next, for 'tis time, my muse declares and sings through the differently painted surfaces, produce distinct tints of brown or black, sensibly differing in intensity, according to the shades of the picture, and where the light is unaltered the colour of the nitrate becomes deepest. When the shadow of any figure is thrown upon the prepared surface, the part concealed by it remains white, and the other parts speedily become dark. For copying paintings on glass, the solution should be applied on leather, and in this case it is more readily acted upon than when paper is used. After the colour has been once fixed upon the leather or paper, it cannot be removed by the application of water, or water and soap, and it is in a high degree perma nent." Mr. Wedgewood endeavoured by repeated washings, and by thin coatings of fine varnish, to prevent the white parts of his pictures from becoming dark when exposed to light; but all his attempts were fruitless, and he was obliged therefore either to exhibit them in candle-light, or for a short time in the shade. This process was applied by its author to taking profiles, and "making delineations of all such objects as are possessed of a texture partly opaque and partly transparent, such as the woody fibres of leaves and the wings of insects." He tried also, but without much success, to copy prints; and he failed still more signally in what was his leading object, to copy the images in the camera-obscura. In following these processes, Sir H. Davy found that the images of small objects produced by means of the solar microscope, may be copied without difficulty on prepared paper-the paper being placed at but a small distance from the lens ;" and he ascertained that about 1 part of nitrate to about 10 of water, gave the best solution. Mr. Wedgewood likewise ascertained that the muriate was more susceptible than the nitrate of silver, and that both were most readily acted upon while wet. He impregnated his paper with the muriate, either by diffusing it through water, and applying it in this form, "or by immersing paper moistened with the solution of the nitrate in very diluted muriatic acid." The impossibility of removing the colouring from the white parts of the pictures, suggested to Mr. Wedgewood the idea that "a portion of the metallic oxide abandons its acid to enter into union with the animal or vegetable substance, so as to form with it an insoluble compound," and he had experiments in view to discover some substance that could destroy this compound either by simple or complicated affinities. Nothing," he adds, "but a method of preventing the unshaded parts of the delineation from being coloured by exposure to the day, is wanted to render the process as useful as it is elegant." 66 This beautiful process, which notwithstanding its defects, it Discoveries of Mr. H. Fox Talbot. 469 required neither science nor skill to repeat, seems to have excited no interest whatever. The writer of this Article gave a notice of it in a Scottish Journal, so early as 1803, but he has not been able to learn that the experiment of Mr. Wedgewood was repeated. Without knowing what had been done by Mr. Wedgewood, Mr. Henry Fox Talbot, of Lacock Abbey, was led by accidental circumstances to turn his attention to the subject of giving a permanent existence to those beautiful but evanescent pictures, which the camera-obscura presents to our view. Recollecting that nitrate of silver was changed or decomposed by light, he began, early in 1834, that series of experiments which led him to the beautiful art which now bears his name. Anxious to perfect the new art which he had discovered, Mr. Talbot continued his experiments till the year 1839, when he communicated to the Royal Society 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. In this paper, which was read to the Society on the 31st January 1839, several months before M. Daguerre had published his photogenic processes, Mr. Talbot enumerates the various purposes to which the new art could be applied; but it was not till the 21st February that he communicated to the Society his process for preparing the paper, and his method of fixing the images. A sheet of superfine writing paper (of a good firm quality and smooth surface) is dipped into a weak solution of common salt (muriate of soda) and wiped dry. A solution of nitrate of silver, namely, a saturated solution six or eight times diluted with water, is spread with a brush over one surface only, and the paper when dry is fit for use. When leaves of flowers, lace, engravings, &c., are laid upon the nitrated surface of the paper and exposed to the sun, very perfect images of them are obtained, the lights and shades being reversed, or, what is the same thing, the pictures are delineated by white in place of black lines, or are negative pictures. In like manner, the pictures thrown upon the nitrated paper placed in the focus of a camera-obscura are negatively delineated. In order to fix these pictures, or prevent the white lines and portions from being blackened by exposure to light, Mr. Talbot first washed them with iodide of potassium greatly diluted with water; but the method which he proposed, as being safer and simpler, was to immerse the picture in a strong solution of common salt, and then to dry it after wiping off the superfluous moisture. At this period Mr. Talbot's pictures were negative, like those of Mr. Wedgewood, but yet he has distinctly shown how positive pictures, or those in which the lights and shades are given as in nature, may be obtained. "In copying engravings," says Mr. Talbot, "by this method, the lights and shadows are reversed, consequently the effect is wholly altered. But if the picture so obtained is first preserved (fixed) so as to bear sunshine, it may be afterwards itself employed as an object to be copied, and by means of this second process the lights and shadows are brought back to their original disposition. In this way we have indeed to contend with the imperfections arising from two processes instead of one; but I believe this will be found merely a difficulty of manipulation."* The communications of Mr. Talbot to the Royal Society could not fail to draw the attention of philosophers to so curious an art, and we accordingly find that the Rev. J. B. Reade, F.R.S., a gentleman to whom the sciences owe valuable obligations, had made important additions to the photogenic processes, and had himself applied them to the delineation of objects of natural history, of which he took pictures by the solar microscope. The following process was communicated by Mr Reade, on the 9th of March 1839, to E. W. Brayley, Esq., who explained the process and exhibited the drawings referred to at one of the soirées of the London Institution on the 10th April 1839. "The more important process, and one probably different from any hitherto employed, consists in washing good writing paper with a strong solution of nitrate of silver, containing not less than 8 grs. to every drachm of distilled water. The paper thus prepared is placed in the dark, and allowed to dry gradually. When perfectly dry, and just before it is used, I wash it with an infusion of galls prepared according to the Pharmacopeia, and immediately, even while it is yet wet, throw upon it the image of microscopic objects by means of the solar microscope. "It will be unnecessary for me to describe the effect, as I am able to illustrate it by drawings thus produced. I will only add, with respect to the time, that the drawing of the flea was perfected in less than five minutes, and the section of cane, and the spiral vessels of the stalk of common rhubard, in about eight or ten minutes. These drawings were fixed by hyposulphite of soda. They may also be fixed by immersing them for a few minutes in weak salt and water, and then, for the same time, in a weak solution of hydriodate of potash. The drawing of the Trientalis Europea was fixed by the latter method: it was procured in half a minute, and the difference in the colour of the ground is due to this rapid and more powerful action of the solar rays. This paper may be successfully used in the camera-obscura. "Farther experiments must determine the nature of this very sensitive argentine preparation. I presume that it is a gallate or tannate of silver, and, if so, it will be interesting to you to know that what has hitherto been looked upon as a common chemical compound is * London and Edin. Phil. Mag. March 1839. No. 88, vol. xiv. p. 208. Mr. Talbot's Double Process. 471 produced or suspended at pleasure by our command over the rays light." of This process cannot fail to be considered as highly honourable to the ingenuity of Mr. Reade. The first public use of the infusion of nut-galls, which, as we shall see, is an essential element in Mr. Talbot's patented process, appears to be due to Mr. Reade, and his process of fixing his pictures by hyposulphite of soda, which has since been universally used as the best, and was afterwards suggested in 1840 by Sir John Herschel, must be regarded as an invaluable addition to the photographic art. Notwithstanding the great beauty of the drawings which Mr. Talbot obtained by the process which he published, the art was still far from being perfect. The discovery of a paper highly sensitive to light was essentially necessary to the production of portraits from the life, and even of accurate pictures of buildings and landscapes, in which the lights and shadows are constantly changing both from the motion of the sun and of the clouds. Mr. Talbot accordingly directed himself anew to this part of his subject, and he succeeded beyond his most sanguine expectations. He discovered a process by which paper could be made so sensitive that it was darkened in five or six seeonds when held close to a wax candle, and gave impressions of leaves by the light of the moon. To this most important invention Mr. Talbot gave the name of Calotype, which his friends have now changed into the more appropriate name of Talbotype, and he secured the exclusive privilege of using it by a patent for England, which was sealed on the 8th February 1841. The following is the patent process for obtaining the negative picture :-Take a sheet of paper with a smooth surface and a close and even texture (and without the water-mark), and wash one side of it, by means of a soft camel'shair brush, with a solution composed of 100 grains of crystallized nitrate of silver dissolved in 6 oz. of distilled water, having previously marked with a cross the side which is to be washed. When the paper has been dried cautiously at the fire, or spontaneously in the dark, immerse it for a few minutes (two minutes át a temperature of 65°,) in a solution of iodide of potassium, consisting of 500 grains to one pint of distilled water. The paper is then to be dipped in water, and then dried by applying blotting paper to it lightly, and afterwards exposing it to the heat of a fire, or allowing it to dry spontaneously. The paper thus prepared is called iodized paper, and may be kept for any length of time in a portfolio not exposed to light. When a sheet of this paper required for use, wash it with the following solution, which we shall call No. 1,-take 100 grains of nitrate of silver dissolved in |