Among the many curious instances of natural magic, none are more remarkable than an experiment not long since proposed, by which Daguerreotype pictures may be taken in absolute darkness to the human eye. This is effected in the following manner:—A large prismatic spectrum is thrown upon a lens fitted into one side of a dark chamber; and as we know that the actinic power resides in great activity beyond the violet ray, where there is no light, the only rays which we allow to pass the lens into the chamber are those which are extra-spectral and non-luminous. These are directed upon, any white object, and from that object radiated upon a highly sensitive plate in a camera obscura. Thus a copy of the subject will be obtained by the agency of radiations which produce no sensible effect upon the optic nerve. This experiment is the converse of those which show us that we may illuminate any object with the strongest sunlight which has passed through yellow glass, the yellow solution of sulphuret of calcium, or of the bichromate of potash—these being non-transparent to the chemical rays—and yet fail to secure any Daguerreotype copy of it, even upon the most exquisitely sensitive plate. Indeed, the image of the sun itself, when setting through an atmosphere which reduces its light to a red or rich yellow colour, not only produces no chemical change, but protects an iodized plate from it; and whilst every other part of the tablet gives a picture of surrounding objects in the ordinary character, the bright sun itself is represented by a spot upon which no change has taken place.[124] In tropical climes, where a brilliant sun is giving the utmost degree of illumination to all surrounding objects, all photographic preparations are acted upon relatively more slowly than in the climate of England, where the light is less intense. As a remarkable instance of this fact, a circumstance may be mentioned, which is curiously illustrative of the power of light to interfere with actinism:—
A gentleman, well acquainted with the Daguerreotype process, obtained in the city of Mexico all the necessary apparatus and chemicals, expecting, under the bright light and cloudless skies of that climate, to produce pictures of superior excellence. Failure upon failure was the result; and although every care was used, and every precaution adopted, it was not until the rainy season set in that he could secure a good Daguerreotype of any of the buildings of that southern city.
The first attempts, which were made at the instigation of M. Arago, by order of the French Government, to copy the Egyptian tombs and temples, and the remains of the Aztecs in Central America, were failures. Although the photographers employed succeeded to admiration in Paris, in producing pictures in a few minutes, they found often that an exposure of an hour was insufficient under the bright and glowing illumination of a southern sky.
Experiments with the spectrum have been made in different latitudes, and it is found, that, as we proceed towards the equator, a band which is always left unchanged, corresponding exactly with the rays of greatest illuminating power, regularly enlarges in size, thus proving the increase of light over actinism—and the interfering power of the former.
By increasing the sensibility of the photographic preparation, this difficulty is overcome, and particularly when any organic compound enters into the preparation. So that we are now enabled to copy nature in all her varying moods, whether we employ our photographic tablets in temperate Europe, or in tropical Africa.
The degree of sensibility which has been attained is remarkable. Mr. Fox Talbot, by uniting a process devised by Dr. Woods, of Parsonstown, and another which was first introduced by the author of this volume, and combining them with an ether, obtains a most unstable compound, which he thus employs. A glass plate is covered with albumen united with the above solution, and then with nitrate of silver: this forms the sensitive surface. The plate being placed in the dark, in a camera, it is so adjusted that the image of a printed bill fixed upon a wheel may fall upon it when uncovered, and the wheel illuminated. The wheel is made to revolve with the utmost rapidity, in a perfectly dark room, and the sensitive plate uncovered. Then the whirling bill is illuminated for an inappreciably short space of time by the discharge of a Leyden jar. Notwithstanding the rapid rate at which the pointed paper is moving, and the instantaneous nature of the illumination—a miniature flash of lightning—the bill is found to be copied with unfailing fidelity upon the photographic plate. It unfortunately happens, that the preparation by which this extraordinary degree of sensibility is obtained, is very uncertain in its action—and hence it is not generally useful; but here we have the evidence to show that at a speed as rapid as that of a rifle-ball an impression may be made upon a photographic plate. There are, however, some new processes which promise eventually to rival the above for sensibility, and to be by no means of difficult manipulation. Of this character is the collodion process. The gun cotton dissolved in ether possesses some very great accelerating properties, and in combination with the silver salts, and one of the vegetable acids, it forms a sensitive surface upon which pictures may be obtained in less than a second of time.
Colour, natural colour too, has been very decidedly secured. The sun has been solicited to display his palette, and the answer has been a picture in which colour for colour in all their fidelity have been impressed. The plate upon which this result has been obtained is of a dark brown colour, and the chromatic variety is, as it were, eaten out by the solar rays. These colours have not yet been permanently fixed upon the plate employed, but from the temporary degree of fixedness which has been obtained, we may fairly hope that in a short time colour may be rendered as permanent on the productions of the photographer as on those of the painter. It is a curious and striking fact, that in the preparation of these plates, salts are used which give colours to flame; and according to the colour which is produced by them when burning, so, on the photographic plate, is that colour impressed with greater intensity than the others. To what is this leading us? Mysteries surround our advances on the domain of truth. We dare not speculate upon them: the time of their full development will arrive.
By the aid of this beautiful art, we are enabled to preserve the lineaments of those who have benefited their race by their intellect, or their heroism. We can hand down to future ages portraits of our own Wellington, and the illustrious Arago, unerring in their truthfulness. How great would be the joy of all, could we now obtain a daguerreotype portrait of a Greek poet, or of a Roman philosopher, of a Sophocles, or of a Seneca! How much discussion would be prevented did we possess a calotype portrait of the Bard of Avon, or of the Philosopher of Grantham!
By the agency of those very rays which give life and brilliancy to the laughing eye and the roseate cheek, we can at once correctly trace the outline of the features we admire, with all those shadowy details which give a reality to the “presentment.” The objects of our love may be for ever present with us in these self-painted pictures. The vicious, whom we would avoid, may be made known to us by this unerring painter. The process which nature employs is perfect; the imperfections are those of man, and these being few, he may soon learn to remedy.
To the traveller, how valuable are the processes of photography! He secures representations of those remains of temples which were in their glory when Moses wrote. He copies by one operation a tomb at Karnac, covered with myriads of hieroglyphics, or an inscribed stone in Arabia, which it would occupy him days to trace. These he can carry to his home and read at his leisure. The relics of hoar antiquity speaking to the present of the past, and recording the histories of races which have fleeted away like shadows, are thus preserved to tell their wondrous tales.