November 20th, 1862, Mr. Chacornac could no longer find the least trace of this nebula, while the small star upon which it was projected did not present any variation of brightness; and the nebula has since been invisible with the instruments of the Imperial Observatory of Paris. We see by the preceding Notices what a degree of activity and interest the researches upon nebulæ now inspire, and also how many difficulties they present. It will not probably be very long before we may hope to obtain a solution of some of the important questions to which they have given rise. ART. XVIII.-On the action of very weak Electric Light on the Iodized Plute; by OGDEN N. ROOD, Professor of Physics in Columbia College. ABOUT three years ago Dove received from Mr. Gunther of Berlin a photograph of a bronze statue of an amazon holding a lance in a perpendicular position, Mr. G. at the same time calling his attention to a singular mark in the picture, which was not in the original. The lance was properly delineated on the negative plate, but in addition, just at its tip, a dark streak was visible, though nothing of the kind had been observed at the time of taking the picture. Careful examination of the plate showed two other analogous marks. Dove thought that these singular appearances might have been caused by the presence of invisible electric brushes, resting on these points, and undertook some experiments to determine whether weak electric light could be photographed. Geisler tubes were used in a dark room, and with the aid of Gunther, he succeeded in obtaining good photographs of the stratified discharge,' as Prof. Wm. B. Rogers' had done some months previously. This led me to attempt the study of the electric brush by the aid of photography, but as its light is incomparably weaker than that from a Geisler tube, I found that no impression was produced on the sensitive plate. Being unwilling to abandon the matter, a very sensitive collodion was then prepared from pyroxyline, in which the cotton fibre was somewhat disintegrated, and by its use I finally obtained good photographs of the posi tive, as well as of the negative brush. An ordinary camera was employed, and the exposure lasted seven minutes. The minute photographs were then enlarged as usual, and prints made from the enlarged negatives. 1 Pogg. Annalen, vol. cxiii, No. vii, "This Journal, xxx, 887. The positive, electrical brush consists, as is well known, of a short stem with widely branching ramifications; these latter are 1. very faint even in the darkest Negative electric brush. It is well known that the negative brush is much smaller than the posi tive, and it is often spoken of as a star or minute point of light; the photograph, however, shows that this is not the case, but that its structure is analogous to that of the positive brush, only that the ramification begins lower down on the stem, as it were, nearly at its root, as is seen in the wood-cut, fig. 2, which is from a magnified photograph. Positive electric brush. Action of weak electric light on the plate in the presence of daylight. -The Geisler tubes in the physical cabinet of the college enabled me now to put the probability of Dove's suggestion to the test of experiment; some of these were connected with an induction coil and photographed in broad daylight, when it was found that the image formed by the electric discharge could be easily traced through the length of the tubes, and that even the stratification was still partially visible. In these cases, however, the electric light was still visible to the eye during the discharges. Accordingly, to make an exact experiment on this point, a sheet of white paper was placed behind one of these tubes and white daylight reflected through it towards the camera. The intensity of this reflected light was so regulated that the bright envelope of the platinum wire was nearly invisible, and the dif fused violet light, at a greater distance from the wire, absolutely invisible. Nevertheless an intense photographic image of the envelope, and a very distinct image of the diffused electric light was easily obtained, thus proving conclusively the correctness of Dove's assumption, that electric light, which in ordinary daylight could not be seen, owing to its feeble illuminating power, might yet make itself very evident on the iodized plate, by vir tue of its high percentage of chemical rays. This experiment is indeed a very striking proof of the chemical activity of the electric light, the more so, as according to some of my experiments, the iodized plate is by no means as sensitive to slight differences in illumination as the human eye. Among the Geisler tubes belonging to the college I found one in which bulbs of uranium glass were alternated with small tubes of plain colorless glass. When the room was darkened, and the electric discharge passed through it, owing to their fluorescence the balls shone very brightly, invisible or faintly visi ble light being converted into bright green light. On taking a photograph of the tube, it was quite surprising to see how blank were the spaces on the plate, where the images of the green bulbs had fallen; after an exposure of four minutes only one of the bulbs could be faintly traced, though other portions of the discharge were represented by an intense deposit of silver. This shows how completely the electric light is divested of its chemical power by dispersion from a thin stratum of this kind. of glass. It might be supposed that uranium glass would cut off most of the chemical rays, when ordinary daylight was transmitted through it, but this was not found to be the case. I placed the Geisler tube with the uranium bulbs, so that the light from a bright sky fell directly through it on the lens of the camera, the entire aperture of the lens (a "portrait combination" of six inches focal length) was used, and the exposure lasted one minute. An examination of the negative plate showed that the thin walls of the uranium bulbs had merely diminished to some extent the chemical power of the rays passing through them. The same experiment with a plate of uranium glass two-tenths of an inch in thickness gave a result like in kind only differing in degree: the chemical intensity of the light being diminished about one-half. This shows, in accordance with theory, that it is mainly the dispersed light which has lost its chemical power, and that through a plate of even this thickness many chemical rays still penetrate. A photograph of another Geisler tube, in which the interior discharge tube was surrounded by a solution of sulphate of quinine, was also taken: this liquid by its fluorescent property diminished, of course, the intensity of chemical action of the electric light, but by no means to the same extent as the uranium glass. Feb. 3d, 1864. ART. XIX.-On the Invisibility of Nebulous Matter; by It has generally been supposed that if nebulous matter, in the proper sense of the word, or cosmical vapor, exists in the heavens, and within reach of our telescopes, it will be visible to the eye, with suitable optical aid. It is proposed to show in this article, with some plausibility, that this is an erroneous idea, except in some particular cases. Comets are the only celestial objects, whose physical constitution is approximately understood, that afford us anything like a distinct notion of what nebulous matter is. By far the greater proportion of these bodies are composed of materials so extremely rare that the solar rays can penetrate completely through the denser portion of their bodies, and the light in some cases seems to suffer scarcely any diminution in intensity. Yet these bodies, which perhaps would weigh at the surface of the earth but a few ounces, or but a few pounds, are distinctly visible with the smallest optical aid, and even, under favorable circumstances, with the naked eye. Sir John Herschel says, of this class of comets, that the most unsubstantial clouds which float in the higher regions of our atmosphere, must be looked upon as dense and massive bodies in comparison with the almost spiritual texture of these light bodies. A cloud composed of materials so rare, and whose distance from us did not exceed fifteen or twenty miles, would scarcely be visible. A comet, however, will be visible when its distance from us is many millions of miles. What conclusion can we draw from these facts? Do they not indicate that comets do not shine wholly by reflected light? On the 3d of July, 1819, Arago made an attempt to analyze the light of comets, by applying his polariscope to the great comet of 1819. The instrument showed unmistakable signs of polarized light, and, therefore, of reflected sun-light. Similar observations on Halley's comet, in 1835, more clearly indicated the existence of polarized light. "These beautiful experiments still leave it undecided, whether, in addition to this reflected solar light, comets may not have light of their own. Even in the case of the planets, as, for instance, in Venus, an evolution of independent light seems very probable." "The variable intensity of light in comets cannot always be explained by the position of their orbits, and their distance from the sun." After mentioning Arago's observations, with his polariscope, on Halley's comet, in 1835, Mr. Hind says, "Still the variation in the intensity of light is not universally such as should follow if the comet merely reflected the sun's rays under 1 Cosmos, vol. i, pp. 90, 91. Bohn's edition. Cosmos, vol. i, p. 91. 1 2 3 certain permanent conditions, and we are under the necessity of looking to physical causes inherent in the body itself for an explanation of some few observations which appear irreconcilable with the theory of reflected solar light." The molecular conditions of the head or nucleus, so seldom possessing a definite outline, as well as the tail of the comet, is rendered so much the more mysterious from the fact that it causes no refraction." 4 I have collected these facts together to show that reflected solar light cannot completely explain, at present, all the phenomena of the light of comets. Besides the above observations, it may be added that the visibility of comets in the day time, and even when near the sun, also indicates a light-generating process in the comet; for otherwise we must suppose them capable of reflecting more light than the planets. Indeed, it is difficult to see how a body can maintain its gaseous or nebulous state without being kept at a high temperature, and therefore, having within itself a light-producing cause. Assuming, then, that there is a light-generating process that is active in comets, let us see what use can be made of it, and whether it will afford us any light on the subject of the visibility of nebulous matter. It is now a well-established fact that the heads of comets contract in dimensions as they approach the sun. This was first noticed by Hevelius, but was not established till many years afterwards. What is the cause of this condensation of cometary matter as the comet approaches the sun? Whatever may be the cause of it, we know that it has a great effect on the visibility of the comet. Is it not possible that the solar rays, acting chemically or otherwise, excites in the comet those principles which cause it to send out in greater abundance, direct light? We know that a comet will increase in brightness with great rapidity as it approaches the sun; and also decrease in brilliancy with equal rapidity in general, as it recedes from the sun, so that the fainter comets disappear in the best telescopes, when, their apparent dimensions only being considered, they ought to remain visible. The dilatation of the cometary volume seems to prevent the comet from sending out much light, either reflected or direct. Applying these principles to nebulae proper, we must conclude that nebulous vapor is necessarily too diffuse, has too little density to be visible, when far removed from us. According to this, then, nebulæ cannot in general be visible, unless they are considerably condensed, and perhaps actually converted into stars. It is perfectly evident that the luminosity of nebulous vapor must be very feeble, even where the light is inherent. The process of condensation only, then, can render nebulous matter visible through our telescopes. Will not this account for the fact that 'Hind's Treatise on Comets, p. 24. • Cosmos, vol. iv, p. 548. |