Some advances have been made towards producing Photographic impressions in color--the impossibility of which some of our best and oldest artists have most pertinaciously maintained. The colored image of the spectrum has been most faithfully copied, ray for ray, on paper spread with the juice of the Cochorus Japonica, (a species of plant) and the fluoride of silver; and on silver plate covered with a thin film of chloride. The day may be still remote when this much to be desired desideratum shall be accomplished in portrait taking; but I am led to hope that future experiments may master the secret which now causes it to be looked upon, by many, as an impossibility. That great advantages have resulted, and that greater still will result from the discovery of the Photographic art, few will deny. The faithful manner in which it copies nature, even to the most minute details, renders it of much value to the painter; but a few minutes sufficing to take a view that formerly would have occupied several days. Its superiority in portraits, over miniature or oil painting has been tacitly acknowledged by the thousands who employ it to secure their own, or a friends likeness, and by the steady increase in the number of artists who are weekly, aye daily springing up in every town and village in the land. CHAP. III. SYNOPSIS OF MR. HUNT'S TREATISE ON "THE INFLUENCE OF THE SOLAR RAYS ON COMPOUND BODIES, WITH ESPECIAL REFERENCE TO THEIR PHOTOGRAPHIC APPLICATION." OXIDE OF SILVER exposed for a few hours to good sunshine, passes into a more decided olive color, than characterises it when first prepared by precipitation from nitrate of silver. Longer exposure renders this color very much lighter, and the covered parts, are found much darker, than those on which the light has acted directly. In some instances where the oxide of silver has been spread on the paper a decided whitening process in some parts, after a few days exposure, is noticed. Oxide of silver dissolved in ammonia is a valuable photographic fluid; one application of a strong solution forming an exceedingly sensitive surface. The pictures on this paper are easily fixed by salt or weak ammonia. NITRATE OF SILVER.--This salt in a state of purity, does not appear to be sensibly affected by light, but the presence of the smallest portion of organic matter renders it exceedingly liable to change under luminous influence. If a piece of nitrated paper is placed upon hot iron, or held near the fire, it will be found that at a heat just below that at which the paper chars, the salt is decomposed. Where the heat is greatest, the silver is revived, and immediately around it, the paper becomes a deep blue; beyond this a pretty decided green color results, and beyond the green, a yellow or yellow brown stain is made. This exhibits a remarkable analogy between heat and light,--before spoken of in chap. II--and is of some practical importance in the preparation of the paper.