Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XLIII.
HELIOCHROMY, OR THE ART OF TAKING PHOTOGRAPHS IN NATURAL COLORS.

SIR JOHN HERSCHEL observed in 1840, that. paper, prepared with chloride of silver and blackened in the sun, when ex posed beneath red or blue glass, assumed the respective color of the glass; and Edmond Becquerel in 1847 and 1848 produced all the colors of the spectrum on a prepared silver plate, which were permanent as long as the plate was kept in the dark. The surface was sensitized by immersing the plate either in a solution of a bichloride, or in chlorine ´eater.

Niepce de St. Victor, following in the steps of Becquerel, is obstinately persevering in his attempts to fix the colors which can already be obtained. The production of colors is a fact; the fixation of colors is still a problem unsolved.

The plate for heliochromic purposes is best prepared in the following manner: A common daguerreotype plate is varnished on the copper side. In one corner a hole is bored in order that the plate can be suspended on a silvered copper wire, which is the positive pole of a galvanic battery. A small plate of platinum soldered to a copper wire forms the negative or zinc pole. As soon as the battery is in working condition, insert the two poles at the proper distance in a vessel containing a mixture of one part of hydrochloric acid and eight parts of water. By the electrolytic decomposition hydrogen affects the negative pole and is given off there, whereas chlorine goes to the silvered plate, combines with the silver, and forms a chloride. The operation is best performed in the dark-room. The amount of deposition is recognized by the different shades of color which the plate assumes. The thinner the film, the more sensitive it is.

Niepce de St. Victor has recommended the production of this sensitive film by immersing the plate in soluble bichlorides, or in chlorides in combination with copper salts.

In. whichever way prepared, the plate is dried over the flame of a spirit-lamp, and the surface is gently brushed over with a tuft of cotton, in order to remove a downy substance arising from impurities.

If this plate be exposed to a diffused light, the film, assumes a grayish violet tinge; but if it, be exposed to a well-defined and very luminous spectrum, it receives an impression of the various colors of this spectrum, but not with the wine facility. The orange, the yellow, and the red are the first colors that appear, anal the first to darken and to become gray by a continued action. Beyond the red a rosy hue is made manifest, but this darkens the first of all. The blue, the green, and the violet are the most vivid. Beyond the termination of the violet part of the spectrum, there is a decomposition of a gray color.

By keeping the prepared plates exposed to a temperature of about 100° or forty-eight hours in a stove, or to the rays of the sun beneath a piece of red glass, the film becomes much more sensitive, and not only reproduces the spectrum but receives an impression from white light.

If, when the plate leaves the electrolytic bath, it be simply dried, without raising the temperature to such a degree as to change the color of the: film, and after this the plate be exposed beneath a colored engraving, a reproduction of these colors will soon be effected; some of these are sometimes latent, whilst others are brilliantly manifest. Those which are latent can be developed by simply rubbing the surface gently with a tuft of cotton impregnated with ammonia, which has been previously used for cleaning a plate. It is hence evident that a colored image is already produced, which may be partially manifest and partially latent.

Two very important problems remain to be solved: to find means of developing the whole of the image at once in all its colors, and of fixing it when developed. Sonic colors can be always reproduced, whilst others are but partially obtained. None of the colors as yet can be rendered permanent in diffused light. This branch of photography, therefore, is still quite imperfect. It is difficult to form an opinion as to the possibility of the solution of this interesting problem; because as yet no clue, no rational hypothesis can be given of the cause of the reproduction of the colors in question. In the ordinary positive printing on the chloride of silver, the cause of the decomposition is probably just as little understood; but we are satisfied with almost any theory as long as the manipulation is definite in its management, and within our power to continue or restrain. In the reproduction of colors these characters the wanting, and we are hence tempted to disbelieve in the possibility of the effectuation of so desirable a discovery. On the other hard, the very fact that colors can be once reproduced, engenders faith in the realization of the great object; and because, a similar and apparently equally as difficult case of fixation of a fugitive image has already been overcome, hope still points to the goal of final success.

Our knowledge of the image-impressions by contact, by the influence of beat and of electricity, is limited simply to the recorded facts, for which as yet no satisfactory rationale bas been assigned. Probably all the pictorial representations of objects in photography, and its congene branches, developed either by mercury, pyrogallic acid, the protosalts of iron, the breath, impalpable powders, etc., may be classified in one and the same category, of which the cause may either be a molecular or polar change-that, is, either an absolute change of position of the ultimate atoms in the aggregated material, or simply a change in the attractions of these atoms. The hypothesis thus expressed is founded upon the circumstance; that every latent image is developed by means of matter applied to the filth, which is attracted to certain parts after exposure by apparent predispositions in these parts, which have been superinduced by this exposure; the resulting pictures are combinations of the new and applied material with the original matter in the film. By an extended course of definite experiments applied, understandingly in reference strictly to cause and effect, we have a right, in the course of tune, to expect a definite solution of these wonderful manifestations of the presence of an Omniscient Intelligence.