ABSORPTION OF SMOKE-COLOURED GLASS. 331 respond with one of Sir John Herschel's heat spots, and may possibly be referred to some peculiar chemical action due to the so called parathermic rays. The presence of vapour, in the form of light cloud or mist, however attenuated, appears to obstruct this peculiar class of rays. (565.) SMOKE-COLOURED GLASS.-Does not appear to alter the colours of natural objects observed through it. Blue of spectrum nearly obliterated, but the indigo and violet rays are extended; entire length of spectrum is not lessened. The most remarkable feature is the way in which the spectrum is extended over the violet end, proving the existence of red rays far down in the ordinary blue rays. (566.) Chemical Spectrum.-Chemical action commences at 70 and extends to 1.90. At the least refrangible end the impression descends faintly to 50. The maximum of action is within the limits of the visible most refrangible rays, the most intense spot being near Fraunhofer's line H. α (567.) A much more extensive series of spectra has been obtained, but those examples only have been selected which appear to illustrate general principles. The deductions to be drawn from the above results are numerous and important, but since it is my intention to devote the concluding division of this work to a philosophic examination of the entire subject, the student is referred forward for the views which I entertain relative to both the luminous and actinic spectra. 332 CHAPTER XII ON THE POLARISATION OF THE CHEMICAL RAYS. (568.) IN paragraph 23. of this volume I have given an experiment described by Dr. Young, which appeared to him to prove the analogy of the "visible and the invisible rays." Dr. Young certainly proves in that experiment, that the chemical principle of the sunbeam is subject to the same laws of motion as the luminous principle, since he shows that these rays are capable of producing the phenomena of interference. M. J. E. Bérard, in a paper Upon the Properties of the different Kind of Rays which are separated by Means of a Prism from Solar Light," which he communicated to the French Institute in 1812, writes: "I received the chemical rays directed into the plane of the meridian, on an unsilvered glass, under an incidence of 35° 6'. The rays reflected by the first glass were received upon a second under the same incidence. I found that when this was turned towards the south, the muriate of silver exposed to the invisible rays, which it reflected, was darkened in less than half an hour; whereas, when it was turned towards the west, the muriate of silver exposed in the place where the rays ought to have been reflected, was not darkened, although it was left exposed for two hours." "It is," says Bérard, "consequently, to be presumed, that the chemical rays can undergo double refraction in traversing certain diaphanous bodies; and, lastly, we may say that they enjoy the same physical properties as light in general." (569.) In a communication to Dr. J. Sutherland of Liverpool, Professor J. Forbes, states: "In spring, 1839, POLARISATION OF ACTINISM. 333 I tried the experiment of letting the picture formed by polarised Light passing through calc spar to fall upon sensitive paper, then newly discovered: whether from the fault of the paper, I know not, but on my first trial I obtained no kind of effect, and, my attention being occupied with other matters, I never repeated it; but at Birmingham, in August 1839, being requested to give some account of the Daguerréotype which I had seen in Paris, I mentioned the experiment, and pointed out its valuable application to fix with unerring accuracy phenomena of diffraction and polarisation, which different eyes have seen differently, and which, regarded as the test of theories, would thus be preserved with unimpeachable fidelity for examination at leisure by every eye." (570.) Dr. J. Sutherland of Liverpool, published in the Philosophical Magazine for July 1811, a paper on this subject, of much interest and considerable importance. Shortly after the publication of that paper, having the advantage of employing a very excellent solar microscope, with polarising adjustment attached, belonging to a friend, I engaged in precisely a similar set of experiments, and obtained the most satisfactory results. At the time of the publication of the first edition of this work, I attached too little importance to these phenomena of polarisation, and therefore, a mere notice of Dr. Sutherland's experiments and of my own is all that appeared. The advances made in the inquiry demand more exact attention to these phenomena; and since too little notice has been taken of Dr. Sutherland's researches, I am induced to extract some of his observations, and connect them with my own results. (571.) A prism of calcareous spar was inclosed in a case, with an aperture at one extremity, through which a concentrated pencil of the most refrangible rays of the solar spectrum from a flint-glass prism were passed, producing two well-marked circular images, which fell, in my experiments, upon paper prepared with iodide of silver and gallic acid. Both images produced some effect, but in nearly all cases-certainly not in all-the extraordinary pencil produced a more decided effect than the ordinary pencil. (572.) I adopted exactly Dr. Sutherland's arrangement, it is therefore due to him that I copy his description, since I obtained precisely the same result: "An analysing apparatus, consisting of six thin plates of mica, was placed obliquely in the course of the polarised ray, so as to form with its axis an angle of about 25°. The instrument was turned round until the plane of the mica plates coincided with the plane of polarisation of the ray. When this was done, the Light was almost extinguished, and was allowed to fall on a piece of photographic paper. After the lapse of five minutes, no effect whatever was produced on the paper. The mica plates were then turned round 90°, until their plane was at right angles with the plane of polarisation. The Light was greatly increased in intensity, and in one minute the paper was tinged, in three minutes a good deal so, and in five minutes was pretty dark." When using a second prism of Iceland spar, and transmitting the polarised beam along it, and turning it on its axis until one of the rays was extinguished, a piece of sensitive paper received a dark image from the unextinguished ray, but the extinguished ray produced no effect whatever. (573.) Having carefully adjusted my apparatus in such a manner that I was enabled to cut off all the rays which have, under ordinary circumstances, any illuminating power, the rays beyond the violet alone were allowed to pass through the Iceland spar. That these dark rays did pass was proved by their making two impressions on the prepared surface placed to receive them. The bundle of mica was then employed as before, and in every experiment the result was the same. (574.) The following result by Dr. Sutherland is instructive. Similar experiments have been recently brought DR. SUTHERLAND'S EXPERIMENTS. 335 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 á 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 |