to distinguish nebula better than the larger instruments. He does not think the variability of the last two nebula of Taurus has been distinctly shown. Mr. D'Arrest, in an article published in No. 1393 Astr. Nachr., acknowledges the variability of the nebula near Merope, and points out another nebula marked by Jeaurat to the north of Pleïone, in a chart of the Pleiades published by him in the Mémoires de l'Académie des Sciences of Paris, for 1779, which has not yet been rediscovered. He presumes, therefore, that this region of the heavens is especially subject to variations of light. The same astronomer has announced, in No. 1407 Astr. Nachr., that Sir John Herschel is preparing a new general catalogue of nebulae from observations both ancient and modern, and he gives on this occasion a list of corrections of the catalogue of 1833, resulting from his own observations and those of Messrs. Auwers and Marth, which Sir John Herschel will be able to make use of in his new work. The later numbers of the Monthly Notices of the Astronomical Society of London contain no articles relating to nebulæ. I will cite only one article by F. Abbott, dated at his private observatory at Hobart-Town, (Australia,) in May, 1862, and published in the Monthly Notices, No. 1, Vol. 23, page 32. This article, presented to the Society at its session Nov. 14, 1862, with a drawing which has not been reproduced in the Monthly Notices, is devoted to a cluster of stars in the Southern Cross designated by the Greek letter Kappa. "This beautiful cluster," says Mr. Abbott, "which Sir John Herschel stated to be composed of 50 or 100 stars of different colors is scarcely visible with the naked eye; but viewed with a telescope it is one of the most brilliant and interesting objects in the southern heavens, not only for the extreme beauty of color and arrangement which distinguish it, but also for certain changes which appear to have taken place in the number, position and color, of the stars which compose it, since it was observed at Feldhausen, near the Cape of Good Hope, by Sir John Herschel, about the year 1835." The seventy-five stars found in the drawing of Mr. Abbott have been observed and their positions determined by him, either with a telescope of five feet focal length, furnished with an excellent achromatic object glass of 41 inches diameter, or with an achromatic telescope of seven feet, made by Dolland. The magnifying power employed for determining their positions was one hundred and thirty-five, and that for the colors only twenty-seven diameters. Many of the stars appear to have changed their positions, and a considerable number of those figured in the drawing of Mr. Abbott are not found in the description and representation made by Sir John Herschel. AM. JOUR. SCI.-SECOND SERIES, VOL, XXXVI, No. 110.-MARCH, 1864. Several of the stars have preserved their color, but according to Mr. Abbott, most of them have changed; all the small stars, from the tenth to the fourteenth magnitude, have the color of Prussian blue, with more or less of a tint of red or green with the blue. mixed Mr. Bond, of Cambridge, U. S., continues his observations upon nebula. According to a note added to the English translation of my review of this subject, in September, 1862, (published in the American Journal of Science, for January, 1863,) this astronomer is now preparing to publish a complete account of fourteen years observations upon the nebula of Orion made with the telescope of Harvard College; and the comparisons which will result will very probably furnish new and interesting remarks. In another note, the editors of the American Journal say that they are authorized to state, contrary to an assertion of Otto Struve, reported in my Notice, that Mr. Bond has distinctly recognized and recorded, in five original sketches drawn in 1847 and 1848, the nebulous bridge, situated upon the Sinus Magnus in one of the regions of the nebula of Orion, and that this bridge is figured in most of his more recent sketches. No. 1383, Astr. Nachr., contains a new plate upon a small scale of this same nebula of Orion, and also numerous stars situated upon and around it, drawn in 1861, by W. Tempel, from his own observations made with a telescope of four inches aperture, with magnifying powers of only 20 to 40 diameters. Finally, I will report a note by Chacornac, entitled "Nébuleuse variable de du Taureau," presented by Mr. Le Verrier to the Academy of Sciences of Paris at its session April 6, 1863, and inserted in the Comptes Rendu of that meeting, t. 56, p. 637. Mr. Chacornac, at Marseilles, in the latter part of 1853 and the first part of 1854, noticed a star of the eleventh magnitude situated at about 5h 28m of right ascension and 21° 7' of northern declination, without perceiving any nebula at that point; but he could not see any at Paris near the meridian towards the end of 1854, with a telescope of twenty-five centimeters aperture, although the atmosphere was very transparent. The 19th of October, 1855, he observed a faint nebula projecting itself upon this same small star very near to of Taurus. November 10th, the nebula had changed neither its place, dimensions, or form. January 27th, 1856, it appeared to him quite brilliant, presenting the appearance of a transparent cloud which seemed to reflect the light of of Taurus; its appearance, totally different from that of the nebula No. 357, (Herschel, 2,) gave no idea of stellar points visible throughout the whole extent of its surface. It was like a light cirro-stratus, striped with parallel bands, presenting a form nearly rectangular, the greater side of which measured an arc of 3 minutes, and the smaller arc 2 minutes. 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 Todized 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 positive, 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, 387. 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 2. 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 chem ical 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-balf. 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. |