upon the chemical constitution, and consequently, the sensibility of the preparation. The character of the bath from which the silver is constantly removed by the immersion of the iodized collodion on the glass, is constantly varying, and even the iodized collodion itself is liable to change, thus producing alterations in the sensitive surface, which, though inappreciable in the ordinary process of obtaining a picture, become well marked when we endeavour to impress it with an image of the prismatic spectrum.

(234.) The normal spectrum was formed by a very pure flint-glass prism. Light admitted between two knife-edges, separated th of an inch, and generally passed through a hole of the same diameter in an inner screen. The chromatic image was received on a white tablet in a perfectly black box; its length, when most accurately adjusted, was 1 inch and ths, which space has been divided into 100 parts; and to this division the numerical details refer. Chemical action commences 40 above the lower end of red, and from this point extends to the length of 11⁄2 inch. Over the space covered by the red and orange rays are indications of a well-defined circle of protective action; immediately above this a dusky brown commences, forming a kind of fringing, which is extended to '60, and in a similar manner it bounds the whole of the spectrum. This is due to diffused light, which I always find bordering the spectrum. Over a space equal to 10 a welldefined black space appears; then the action weakens, but is still strong over 7, when it again increases just at the end of the violet, and is somewhat sharply cut off at 1.90 above 0, or lowest red, presenting an image similar to that represented in the margin. Placing the glass at a small angle, and observing the spectrum by reflexion, the lower dark space comes out very strongly, and the whole space

above it appears of a dark semi-metallic purple shaded by a dusky brown border. It is quite impossible to represent these delicate differences between the respective portions by a wood engraving. The accompanying figure is, therefore, to be regarded as representing with accuracy the general character of the impression, showing the limits of greatest and of least action, and little more.

(235.) By operating with a well-defined spectral image of a small slit, formed by the knife-edges, throwing this spectrum upon a large lens of long focus, and receiving the image at the distance of several feet from the lens on the collodion plate, we obtain a spectrum showing most of the dark lines of Fraunhofer, particularly those which are discovered at the most refrangible end of the spectrum, and over the space beyond the luminous rays, which is rendered luminous by the fluorescent rays of Professor Stokes, these being so many inactive spaces, which are in the first impression represented as so many white lines. A positive copy being taken from the negative spectrum, these are then represented correctly as dark lines, with an inversion of the character of the chemically darkened spaces. This, however, proves only that the modes of motion of Light and actinism are the same, it does not prove the identity of these forces. (See frontispiece).

L

146

CHAPTER IV.

SOLAR ACTION ON VARIOUS METALLIC COMPOUNDS.

(236.) GOLD. It has been long known, that a solution of gold in ether decomposes by exposure to the sun's rays, metallic gold being deposited on the side of the vessel nearest the Light. Charcoal saturated with a solution of the chloride of gold, and exposed to the sun's rays, is speedily covered with a very fine film of the revived metal. Ivory may also be gilded by washing it with the same solution and exposure. Most of the salts of gold, indeed, are reduced by solar agency.

(237.) Sir John Herschel, in the Philosophical Transactions, Part I. for 1810, has given some very interesting particulars respecting some aurated preparations:

Papers washed with CHLORIDE OF GOLD, freed from an excess of acid, are slowly changed under the influence of the solar beams, a regularly increasing darkness takes place, and the paper at length becomes purple (Herschel). I have observed that the first action of the Light is to whiten paper, which has been rendered a pale yellow by the wash of chloride of gold. If papers are removed from the Light when thus bleached, it will be found that a darkening action will gradually come on, and eventually develop the picture impressed on the paper. This process is much quickened by placing the paper in cold water, and, however slight the exposure may have been, the process of darkening continues until all the salt of gold is decomposed.

(238.) Chloride of gold with nitrate of silver gives a precipitate of a yellow brown colour, possibly metallic double salts, in which the gold as well as the silver is in the state of chloride. On glass this precipitate is but

very

slightly sensitive; on paper it is blackened somewhat more speedily (Herschel).

(239.) If paper impregnated with oxalate of ammonia be washed with chloride of gold, it becomes, if certain proportions be hit, pretty sensitive to Light, passing rather rapidly to a violet purple in the sun (Herschel). I have found it exceedingly difficult to arrive at the best proportions generally speaking, thirty grains of the oxolate of ammonia, and a saturated solution of the chloride of gold, has been the most successful in practice. These papers must be dried in the dark without heat. After the picture has been obtained, I have succeeded in fixing it, by soaking it in cold water, and then washing it over with the ferrocyanate of potash.

(240.) Paper impregnated with the acetate of lead, when washed with perfectly neutral chloride of gold, acquires a brownish-yellow hue, and a sensibility which, though not great, is attended with some peculiarities highly worthy of notice. The first impression of the sun's rays seems rather to whiten than to darken the paper, by discharging the original colour, and substituting for it a pale greyish tint, which by slow degrees increases to a dark slate colour; but if arrested, while yet not more than a moderate ash grey, and held in a current of steam, the colour of the part acted on by the solar rays (and of that only) darkens immediately to a deep purple: the same effect is produced by immersing it in boiling distilled water. If plunged into cold water, the same change comes on more slowly, and is not completed till the paper is dried by heat. A dry heat, however, does not operate this singular change (Herschel).

(241.) Prismatic Analysis.—Chloride of Gold. The maximum effect is produced by the mean blue ray, and the influence is exerted but a little way below the green; indeed, it is doubtful if it can be said, that any visible effect is produced below the green itself. Above the blue, the action is carried on, but with declining energy, through

the indigo and violet rays, beyond the most refrangible edge, of which no action can be detected.

To distinguish the following processes from the Chrysotype, which will be described in another section, I propose to designate them as Aurotypes.

(242.) Aurotypes. Protocyanide of potassium and gold, prepared according to Himly's method*, was washed over paper and dried; then it was washed with a solution of nitrate of silver, and again dried. This paper darkens with considerable rapidity, and this blackening proceeds steadily in the dark. Good photographs result from this preparation. The pictures are best fixed by soaking in a little salt and water, and then washing with a weak solution of the hyposulphite of soda.

(243.) It will be found that several of the combinations of the oxide of gold with cyanogen yield very interesting pictures, which promise to be of some importance in the photographic art. A few of these may be briefly men

tioned.

(a) Protocyanide of potassium and gold, with a weak wash of nitrate of silver, changes with tolerable quickness, and presents a good contrast of light and shadow.

(b) Protocyanide of gold, formobenzoic acic, and nitrate of silver, give very beautiful results, and are tolerably quick in changing, although as yet no paper has been prepared, sufficiently sensitive for use in the camera obscura. The darkened portions are exceedingly intense, the impression being made nearly through the entire substance of the paper; hence affording very perfect photographs, from which copies of exceeding sharpness may be procured.

(c) Protocyanide of gold, formobenzoate of ammonia, and nitrate of silver, give results of an exceedingly pleasing kind. Papers thus prepared do not appear to be quite so sensitive as those which are prepared with the formo

Appendix No. I.