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.