the interposition of the lens. It indeed appears to me, from many experiments made in this manner, that not only the extreme red ray exerts this very peculiar property, but the ordinary red ray, through nearly its whole length. Including this whitened portion, the whole extent of chemical action exerted is "considerably more than double the total length of the ordinary luminous spectrum. (Herschel.)

(87.) Photographic Application.-It has been already stated, that the chloride of silver was used as a photographic agent by Wedgwood, Davy, and Daguerre, their success however being very limited. This salt became, in the hands of Mr. Fox Talbot, of much importance. As it is to this experimentalist that we are indebted for the first successful application of any chemical preparation on paper, as a tablet on which Light should impress, with unerring fidelity, the objects it rendered visible, it is right that his process should be described in his own words.

"I select," says Mr. Talbot, " in the first place, paper of a good firm quality and smooth surface. I do not know that any answers better than superfine writing-paper. I dip it into a weak solution of common salt and wipe it dry, by which the salt is uniformly distributed throughout its substance. I then spread a solution of nitrate of silver on one surface only, and dry it at a fire. The solution should not be saturated, but six or eight times diluted with water. When dry the paper is fit for use.

"I have found by experiment, that there is a certain proportion between the quantity of salt and that of the solution of silver, which answers best and gives the maximum effect. If the strength of the salt is augmented beyond this point, the effect diminishes, and in certain cases becomes exceedingly small.

"This paper, if properly made, is very useful for all ordinary photographic purposes. For example, nothing can be more perfect than the images it gives of leaves and

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65

flowers, especially with a summer sun: the light passing through the leaves delineates every ramification of their

nerves.

"Now, suppose we take a sheet thus prepared, wash it with a saturated solution of salt, and then dry it. We shall find (especially if the paper has been kept some weeks before the trial is made) that its sensibility is greatly diminished, and in some cases seems quite extinct. But if it is again washed with a liberal quantity of the solution of silver, it becomes again sensible to Light, and even more so than it was at first. In this way, by alternately washing the paper with salt and silver, and drying it between times, I have succeeded in increasing its sensibility, to the degree that is requisite for receiving the images of the camera obscura.

"In conducting this operation, it will be found that the results are sometimes more and sometimes less satisfactory, in consequence of small and accidental variations in the proportions employed. It happens sometimes that the chloride of silver is disposed to darken of itself, without any exposure to Light: this shows that the attempt to give it sensibility has been carried too far. The object is to approach to this condition as near as possible, without reaching it, so that the substance may be in a state ready to yield to the slightest extraneous force, such as the feeble impact of the violet rays when much attenuated. Having therefore prepared a number of sheets of paper, with chemical proportions slightly different from one another, let a piece be cut from each, and having been duly marked or numbered, let them be placed side by side in a very weak diffused light for a quarter of an hour. Then if any one of them, as frequently happens, exhibits a marked advantage over its competitors, I select the paper which bears the corresponding number to be placed in the camera ob

scura.'

* London and Edinburgh Philosophical Magazine, March, 1839, vol. xiv. p. 209.

F

(88.) This variable sensibility of paper, prepared with the same ingredients, differing but very slightly in their proportions, admits of an easy explanation by an experiment. Precipitate upon a clean piece of glass a film of chloride of silver, which is best done in the following manner recommended by Sir John Herschel :-A solution of common salt of extreme dilution, is mixed with nitrate of silver, so dilute as to form a liquid only slightly milky, and into this, at the bottom of a deep vessel, is placed horizontally a glass plate, upon which the chloride is slowly deposited; the liquid being carefully drawn off with a siphon, and the last portions by fibres of hemp. When this is dry, we have a uniform film of the chloride of silver, chemically pure. If we take this plate, and having placed it at a very small inclination, expose it to Light, and drop upon its upper edge a small portion of a solution of nitrate of silver, we shall see, that, as the nitrate solution slowly combines, as it descends, with the chloride, it darkens very unequally; the edges of the descending liquid giving the most rapid indications of sensibility to sunshine. From this we learn that to produce the most sensitive chloridated photographic paper, it is necessary to have an exceedingly slight excess of the nitrate of silver, beyond that which is necessary to effect the entire decomposition of the salt used; but, as I have stated before, this excess is absolutely essential.

(89.) The following are the best proportions, with which my practice has made me acquainted, for producing papers sufficiently sensitive for good pictures by superposition or for positives in general:

Chloride of sodium, fifty grains to one ounce of water; in which solution the paper is washed, and then carefully wiped over with a clean cotton cloth, and dried.

A solution of the nitrate of silver, in the proportions of 120 grains to an ounce of distilled water, is then carefully applied twice over one side of the sheet, drying the sheet between each wash at a little distance from the fire.

(90.) For the less sensitive varieties of this kind of

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67

paper, the silver salt may be used in more economical proportions. Great care is of course required in all photographic manipulation, and the want of attention to the purity of the materials used, their correct proportions by weight and measure, and the absolute cleanness of brushes, cloths, &c. will constantly lead to the most perplexing failures. It may be well to observe in this place, once for all, that the selection of paper for photographic purposes requires to be made with care. The thing to be aimed at is, to obtain as great a transparency as possible, combined with such a thickness as shall ensure perfect opacity in the dark parts of the drawings. It is also to be desired that the sensitive preparations should be retained as much on the surface as possible, for experiment will show, the most striking difference between the same preparation spread on a paper of a firm texture and on an absorptive variety. The best kinds of paper, which are those known to the trade as hand-made and calendered papers, differ considerably on their two surfaces, one being much less absorbent than the other, which is the side that must be chosen as the one for spreading the sensitive washes over.

(91.) It was noticed by Mr. Talbot, in the very outset of his photographic experiments, that however carefully a paper might be prepared with the above materials,-and the same applies to all others,-it would sometimes turn out to be nearly, if not quite, insensible to Light in some parts of its surface. He thus describes this singular quality: 'Exposed to sunshine, this paper will exhibit large white spots of a very definite outline, where the preparing process has failed; the rest of the paper where it has succeeded turning black as rapidly as possible. Sometimes the spots are of a pale tint of cerulean blue, and are surrounded by exceedingly definite outlines of perfect whiteness, contrasting very much with the blackness of the part immediately succeeding." There can be but one opinion as to the cause of this very annoying peculiarity. The chemical compound used, exists in two definite and different states in the light

and dark parts of the paper. We shall find, if we carefully examine the matter, that the same sheet of paper will absorb more moisture in some parts than it will do in others, consequently we shall have (to cite the present case) a larger quantity of salt in some places than in others: and when we apply the nitrate of silver, portions of the paper will become covered with the chloride of silver, having the required excess of the nitrate of silver, while others will consist of the pure chloride, or a double salt, the muriate of soda and silver. Sir John Herschel proposes to remedy this "by saturating the saline washes used, previous to their application, with chloride of silver. (Chloride of sodium will dissolve a considerable quantity of the chloride of silver.) By attending to this precaution, and by dividing the last wash of the nitrate into two of half the strength, applied one after the other, drying the paper between them, their occurrence may be almost entirely obviated." In my own experience I have found this method generally answer the desired object, but it appears to be somewhat injurious to the sensibility of the paper. If the saline wash is applied with a sponge, care being taken not to remove the pile of the paper, it will be found that we are less liable to these spots than when the paper is soaked in the solution.

(92.) A very pretty modification of these processes has been recommended by Sir John Herschel, which resulted from his experiments to ascertain how far organic matter was necessary to produce the change in the chloride and other salts of silver. A film of the chloride is precipitated on a glass plate, in the manner previously described, and carefully dried in the dark. This pure chloride of silver is scarcely sensible to the influence of ordinary daylight, unless by a very prolonged exposure; but if it is washed over with a solution of the nitrate of silver, it becomes exceedingly sensitive, and may be used for receiving pictures in the camera obscura. These are very interesting and well-defined negative pictures, which are direct or reversed according as looked at on the side which was exposed to