"persisted" slightly longest. Likewise, in visual observations at contact II, it would no doubt be the brightest line visible, and "persist" longest. Inasmuch as my moving plate recorded many of the faintest chromosphere lines in Professor Young's list, but made no record of a bright line at A5303, it is pretty certain that the true coronal line would be difficult to observe so long as the chromosphere spectrum was visible. The observers. made it their first duty to fix the position of the green line. The persisting chromosphere line, very conspicuous just before and at the instant of totality, was naturally assumed to be identical with the true coronal line, and its position was fixed at 1474K. Later, when this line had disappeared, rather suddenly, and the background had become dark enough to allow the line at A5303 to be seen, the observers were interested in determining the extent, and other allied properties, of this line; and no further micrometer settings were made for determining its wave-length. This illustrates one of the advantages of photographic methods, now happily available. The photograph above described is not suitable for mechanical reproduction; but, with the Director's assent, I should be willing to supply copies on glass to those investigators who are planning for observations, based on the green coronal line. Professor Young contributed largely to this determination of the position of the coronal line, by arranging for the most generous loan of the four large compound prisms, described above, and belonging to Princeton University; and also, by the fact that the instrument was manipulated during totality, in a perfectly satisfactory manner, by one of his former students in Dartmouth College, the Rev. J. E. Abbott, long a resident of Bombay, who extended many favors to the expedition. LICK OBSERVATORY, This persistence may be due to the greater brightness of the line, rather than to a thicker stratum. THE RING NEBULA IN LYRA. I By JAMES E. KEELER. ON taking charge of the Crossley 3-foot reflector of the Lick Observatory, about a year ago, it was my intention to devote the instrument to spectroscopic work, for which a spectroscope, designed by Professor Campbell, had been partially completed by the Observatory instrument maker. It was first` necessary to make numerous small changes in the mounting and in the guiding apparatus. The pier was also cut down two feet, and a new and powerful driving-clock was made at the Observatory from plans by Professor Hussey. To test the capabilities of the instrument, a number of photographs were then made of well-known celestial objects, and these were of such excellence that I determined to pursue this photographic work for the present, and to put off the spectroscopic investigations until some future time. Among the objects photographed was the ring nebula, which I have chosen for the subject of the present paper; not because it is specially well suited to display the capabilities of the telescope (for this is not the case), but because it is a very wellknown object, to which observers, with photographic telescopes in particular, have paid a great deal of attention, and which possesses in itself many features of interest. In It is very doubtful whether the ring nebula in Lyra has ever been photographed with an entirely suitable instrument. this connection it may be well to recall the fact that the focal length of a camera must be from thirty to sixty times its aperture in order that the photographic and optical resolving powers may be equal. A photograph taken with such an instrument, under perfect atmospheric conditions, should show all that the eye can see. Practically, however, mechanical difficulties of construction, and the faintness of the light emitted by some Read at the Third Conference of Astronomers and Astrophysicists, Sept. 7, 1899. objects, make it necessary to modify the theoretical ratio of focal length to aperture. For photographing faint, diffuse nebulae, in particular, the focal length must be short. In all modern reflectors the focal length is quite small for the aperture. But the ring nebula is photographically a bright object. It is also a small object (80" x 60"), and hence could be most advantageously photographed by a reflector of unusually great focal length. In photographic refractors the ratio of focal length to aperture is usually much greater than in the reflector; but aside from the fact that the absolute focal length of such instruments is, in general, small, the absorption of the chemically active rays by the glass lenses is so great that the nebula can no longer be regarded as a bright object. Thus, I find that exposures of nine, and even up to twenty hours have been given to the ring nebula with. refracting telescopes. With the Crossley reflector such exposures would yield nothing but a large black blotch on the negative. The Crossley photographs.-As an example of the exposuretimes required for the Crossley reflector, I give the following list of negatives made here under the finest conditions: 1899, July 13. Exposure two hours. All parts of the nebula greatly overexposed, though the plate was treated for over-exposure. July 12. Exposure one hour. Over-exposed. July 14. Exposure thirty minutes. Good photograph; treated for The focal length of the Crossley telescope is seventeen and one half feet, and the longer diameter of the ring nebula on the plate is about 2 mm. If the focal length of the telescope were increased four times, the diameter of the image would be about 8 mm, and the length of exposure required would be about three hours, which is not excessive. With such an instrument a far better photograph could doubtless be obtained than any that has yet been made. But it is impracticable to change the focal length of a telescope, while the aperture is easily varied. Considering the Crossley telescope, therefore, as an instrument of fixed focal length, the only advantages to be expected from reducing the aperture are (1) diminution of aberration, (2) diminution of atmospheric disturbance. I. The aberration of a parabolic mirror has been discussed (among others) by Professor Schaeberle, who found it excessive at a short distance from the axis; but the case is not really so bad as he made it out to be, as may be shown by applying the more rigid methods of physical optics to the same problem. The excellent star images on Sir Isaac Roberts' photographs are a practical support of the above statement. With the Crossley reflector the star images are quite good one or one and one half inches from the axis, and at half an inch from the axis they are practically perfect. For such an object as the ring nebula the aberration is therefore insensible. 2. The photographs mentioned in the preceding list were taken on nearly perfect nights, when no improvement in the definition would have resulted from cutting down the aperture. I have tried the Crossley telescope on objects still smaller than the ring nebula. A photograph of the planetary nebula G. C. 4628 (26" × 16"), taken on the night of July 30 with two minutes' exposure, shows the nearly circular outline, the distorted inner bright ellipse, and the central nucleus, almost exactly as drawn by Professors Holden and Schaeberle. The exposure is about right. Another plate, to which was given an exposure of ten minutes, shows the projections or "ansae," strong at the outer ends, and faintly connected with the main nebula, which is greatly over-exposed. For such objects, however, visual observation with the 36-inch refractor is more satisfactory than photography. The photographs of the ring nebula made with the Crossley reflector show features which have been described by observers with powerful visual and photographic telescopes, and others of which I can find no description and which appear to be new. In this connection I have consulted a large number of papers on the nebula, and such drawings and photographs as have been published and are in our library. I have also re-observed the nebula with the 36-inch refractor of this Observatory. The most satisfactory drawing is one made by Professor Holden at Washington in 1875. The original is at the Lick Observatory. Trouvelot's drawing in H. C. O. Annals, Vol. VIII, is also excellent. Neither drawing shows the central star, and both are somewhat too regular and symmetrical. Form of the nebula. - The outline of the ring nebula, as shown by the Crossley photographs, is oval rather than elliptical, the more pointed end being toward the northeast. M. Stratonoff's diagram in A. N., 3388 shows the form well, though the inner dark space is, owing to the long exposure and the spreading of the photographic image, considerably too small. From both sides of the oval project faint masses or fringes of nebulosity, much as drawn by Lord Rosse,' but less uniform in shape and brightness, and having no structure. The most important of these projecting nebulosities are tabulated below, the position angles being measured from the central star as center. Not having made trails on any of the plates, I take the position angle of the bright star following the nebula to be 87.8°, as determined by Professor Burnham.2 |