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EXPOSITION AT VIENNA.
ical observation, and the main reliance of physico-chemical research. It promises to replace all the more tedious processes connected with pictorial reproduction by mechanical ones, at once immeasurably more rapid and more exact; and to spread the cultivation of the plastic arts to an extent certainly never dreamed of by the most devoted enthusiast fifty years ago.
In reviewing the province assigned to me, it seems fitting to divide the Report into two parts,—one on the negative or primary reproduction in which the image is first secured ; and the other on the positive or multiplying methods, in which will be included all the new printing processes. The quest of that philosopher’s stone of photography,—the representation of nature in her actual colors,—so ardently pursued at one time, seems to have been in a great measure lost sight of; and, without assuming such a knowledge of chemistry as should pretend to recognize a limitation of its resources, it may be permitted at least to say, that until new conditions, as yet undreamed of, are developed, this quest must remain a fruitless one. The simple fact that the photographic effect is produced, not by the optical, but by the chemical action of light, would seem to debar us from entertaining any hope that color, which belongs to the former, should ever become an attribute to the latter, in the sense demanded by photography, and the equally unquestionable fact that the subchloride of silver, in course of reduction under the action of light transmitted through colored media, takes, under certain circumstances, somewhat of the color of those media (which is the only phenomenon so far recorded tending to chromophotography), does not necessarily imply the possibility of a sequential reproduction of colors, as this solitary phenomenon may be (and probably is) merely a case of coincident iridescence, the sub-chloride passing through the different primary tints in the course of its further reduction; which conclusion is practically established by the final reduction of the chloride to the usual monochrome by the continual action of light, and the impossibility of rendering permanent the tints so obtained. So far, chemistry has given no hint of a process for practically reproducing color ; and the phenomena of this class which have been produced are, by their fugitive