DR. SUTHERLAND'S EXPERIMENTS.

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forward as original, and, let us hope through ignorance of what had been done, Dr. Sutherland's name has not been alluded to.

"I was next desirous of ascertaining whether any phenomena resembling the coloured rings seen round the axes of crystals in polarised Light, were presented by the chemical rays when polarised; and for this purpose I employed an apparatus consisting of a tube two inches long, and threefourths of an inch in diameter; at one extremity of it was placed a double convex lens, having a focus of one and a quarter inch; within the tube, and at the distance of half an inch from the lens, was placed a section of a calcareous spar rhomb, such as is used for showing the coloured rings. At the other extremity of the tube was placed an oblique analysing bundle of three mica plates, or, one of Nicol's improved prisms, and the apparatus was so disposed that the polarised sunbeam was allowed to fall on the lens, and thence through the tube upon a screen placed close to it. An image of the coloured rings and black cross was thus obtained, and, by turning the tube 90° upon its axis, the rings with the white cross appeared; while in this position a piece of photogenic paper was used to receive the image, and a reverse impression of the rings and cross was obtained; to wit, the place where the white cross had been was dark, the centre light, with a complete black ring round it, and segments of other rings exterior to it. The tube was next turned 90° upon its axis, so as to show an image of the rings with the black cross: sensitive paper was again employed, and another reversed impression obtained; to wit, the position of the black cross was white, the centre and inter-spaces dark, with segments of two or three darker circles on them."

(575.) I have frequently determined the polarisation of the chemical rays by reflexion. A Biot's polariscope has been employed for this purpose. The image of the sun, after having been reflected from the mirror, composed of several plates of parallel glass, was thrown on the analysing

plate, the plane of which had previously been turned at right angles to the plane of primitive polarisation, and from this reflected on a piece of calotype paper; in a few minutes an image was obtained, but this was exceedingly faint. The analysing plate was turned 90° upon its axis, and the polarised beam then thrown on the prepared paper, when, almost instantly, a dark impression was made on the paper. These experiments were subject to several modifications, the results in every case, however, proving the polarisation of the chemical rays by reflexion.

He

(576.) Dr. Sutherland polarised the chemical rays by repeated single refraction, in the following manner. prepared two bundles of mica plates, nine in each bundle. These were arranged diagonally in a tube, one half of which could be turned round within the other. The tube was turned so that the planes of both bundles were at right angles, and the sun's rays were transmitted through it so as to fall on sensitive paper. In a few minutes little cr no effect was produced, but on turning the planes round so as to coincide, an immediate darkening of the paper took place.

(577.) I have constructed an arrangement somewhat similar to this, but using common glass in the place of the mica. The tube containing the polarising bundle was so placed that the sunbeams passed directly through the glass plates to the paper. When the planes were at right angles to each other, the effect, even after prolonged exposure, was weak; but when their planes coincided the paper darkened almost instantaneously.

(578.) By means of a Goddard polariscope and the oxyhydrogen light, I have obtained very beautiful impressions of the rings of Iceland spar, mitre, quartz, and unannealed glass, upon collodion and Daguerréotype plate and iodised paper.

CHAPTER XIII.

ON PRODUCING COLOURED PICTURES BY THE SOLAR

RADIATIONS.

(579.) M. BIOT, in 1840, speaking of Mr. Fox Talbot's calotype pictures, considers as an illusion "the hope to reconcile, not only the intensity, but the tints of the chemical impressions produced by radiations, with the colours of the object from which these radiations emanate." We have produced coloured images-not merely impressions of the rays of the spectrum, but copies-in the camera, of coloured objects, such as geological maps and coloured drawings. As yet these images cannot be fixed with any degree of permanence, but we are advancing rapidly to the desired end.

(580.) It must be remembered that the colour of bodies. depends entirely upon the arrangement of the molecules, constituting the first surface upon which the light falls; and that, by either absorption or dispersion, a chromatic effect is produced. We have numerous very beautiful experiments in proof of this. The biniodide of mercury is a fine scarlet when it is precipitated. If this precipitate is heated between plates of glass, it is converted into crystals of a fine sulphur yellow, which remain of that colour if left undisturbed, but which become very speedily scarlet if touched with any pointed instrument. This curious optical phenomenon has been investigated by Mr. Fox Talbot and by Mr. Robert Warrington. Perfectly dry sulphate of copper is white; the slightest moisture turns it blue. Muriate of cobalt is of a pale pink colour; a very

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slight heat, by removing a little moisture, changes it to a green. These are a few instances selected from a very extensive class of a similar kind.

(581.) If we receive a prismatic spectrum on some sensitive papers, we have evidence that the molecular or chemical disturbance bears some relation to the colour of each ray, or, in other words, that coloured Light so modifies the action of the chemical principle, that the impression it makes is in proportion to the colour of the Light it accompanies, and hence there results a molecular arrangement capable of reflecting colours differently. In the preceding pages, some instances have been given in which the rays impressed, corresponded with the colours of the luminous rays in a very remarkable manner. One of the most decided cases is that of the paper prepared with the fluoride of soda and nitrate of silver. (196.) Sir John Herschel, was, however, the first to obtain any good specimens of photographically impressed prismatic coloura

tions.

It was noticed by Daguerre that a red house gave a reddish image on his iodised silver plate in the camera obscura; and Mr. Fox Talbot observed, very early in his researches, that the red of a coloured print was copied of a red colour, on paper spread with chloride of silver.

(582.) In 1840 I communicated to Sir John Herschel some very curious results obtained by the use of coloured media, which he did me the honour of publishing in one of his memoirs on the subject, from which I again copy.

A paper prepared by washing with muriate of barytes and nitrate of silver, allowed to darken whilst wet in the sunshine to a chocolate colour, was placed under a frame containing a red, a yellow, a green, and a blue glass. After a week's exposure to diffused Light, it became red under the red glass, a dirty yellow under the yellow glass, a dark green under the green, and a light olive under the blue.

IMPRESSED COLOURS.

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The above paper, washed with a solution of a salt of iodine, is very sensitive to Light, and gives a beautiful picture. A picture thus taken was placed beneath the above four glasses, and another beneath four flat bottles containing coloured fluids. In a few days, under the red glass and fluid, the picture became a dark blue, under the yellow a light blue, under the green it remained unchanged, whilst under the blue it became a rose red, which in about three weeks changed into green. Many other experiments of a similar nature have been tried since that time with like results.

(583.) In the summer of 1843, when engaged in some experiments on papers differently prepared, with a view to test the action of gallic acid on various silver salts I had placed in a camera obscura a paper prepared with the bromide of silver and gallic acid. The field of the camera obscura embraced a picture of a clear blue sky, stuccofronted houses, and a green field. The paper was accidentally exposed for a longer period than was intended— about fifteen minutes;-a very beautiful picture was impressed, which, when held between the eye and the Light, exhibited a curious order of colours. The sky was of a crimson hue, the houses of a slaty blue, and the green fields of a brick red tint. Surely these results appear to encourage the hope, that we may eventually arrive at a process, by which external nature may be made to impress its images on prepared surfaces, in all the beauty of their native colouration. Such was the language I employed ten years since when the idea of realising this hope was the subject of doubt and even of ridicule; let us now examine what has been effected in this direction.

(584.) Some years since a gentleman, Mr. Hill, of the United States, published in an American Journal the discovery of a process by which colours were fixed on the Daguerréotype plate. Pictures thus naturally coloured were said to have been exhibited, but the public never had, as far as I am aware, any intimation of the process