Telescopic Work for Starlight Evenings. William F. Denning
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little work was accomplished with it. Latterly its performance has been more satisfactory; excellent photographs of the Moon have been taken, and it has been much employed in observations of nebulæ. The speculum having recently become tarnished, it has been dismounted for the purpose of being repolished.
The silver-on-glass reflector of 47·2-in. diameter, at the Paris Observatory, was used for some years by M. Wolf, who has also had the control of smaller telescopes. He was in a favourable position to judge of their relative effectiveness. In a lecture delivered at the Sardonne on March 6, 1886, he said:—“During the years I have observed with the great Parisian telescope I have found but one solitary night when the mirror was perfect.” Further on, he adds:—“I have observed a great deal with the two instruments [both reflectors] of 15·7 inches and 47·2 inches. I have rarely found any advantage in using the larger one when the object was sufficiently luminous.” M. Wolf also avers that a refractor of 15 inches or reflector of 15·7 inches will show everything in the heavens that can be discovered by instruments of very large aperture. He always found a telescope of 15·7-inch aperture surpass one of 7·9 inches, but expresses himself confidently that beyond about 15 inches increased aperture is no gain.
The Washington refractor of 25·8 inches effected a splendid success in Prof. Hall’s hands in 1877, when it revealed the two satellites of Mars. But immediately afterwards these minute bodies were shown in much smaller instruments; whence it became obvious that their original discovery was not entirely due to the grasp of the 25·8-inch telescope, but in a measure to the astuteness displayed by Prof. Hall in the search. A good observer had been associated with a good telescope; and an inviting research having been undertaken, it produced the natural result—an important success. The same instrument, in the same hands, enabled the rotation-period of Saturn to be accurately determined by means of a white spot visible in December 1876 on the disk of the planet, and which was subsequently seen by other observers with smaller glasses. Good work in other directions has also been accomplished at Washington, especially in observations of double stars and faint satellites. But notwithstanding these excellent performances, Prof. Hall expressed himself in rather disparaging terms of his appliances, saying “the large telescope does not show enough detail.” He gave a more favourable report in 1888; for we find it stated that “the objective retains its figure and polish well. By comparison with several other objectives which Prof. Hall has had an opportunity of seeing during recent years, he finds that the glass is an excellent one.”
Prof. Young, who has charge of the 23-inch refractor at Princeton, has also commented on the subject of the definition of large telescopes. He says:—“The greater susceptibility of large instruments to atmospheric disturbances is most sadly true; and yet, on the whole, I find also true what Mr. Clark told me would be the case on first mounting our 23-inch instrument, that I can almost always see with the 23-inch everything I see with the 9½-inch under the same atmospheric conditions, and see it better—if the seeing is bad only a little better, if good immensely better.” Prof. Young also mentioned that a power of 1200 on the 23-inch “worked perfectly on Jupiter on two different evenings in the spring of 1885 in bringing out fine details relating to the red spot and showing the true forms of certain white dots on the S. polar belt.”
The 26-inch refractor at the Leander McCormick Observatory, U. S. A., is successfully engaged in observations of nebulæ, and many new objects of this character have been found. It does not appear that the telescope is much used for other purposes; so that we can attach no significance to the fact that important discoveries have not been made with it in other departments.
The great Vienna refractor of 27-inches aperture “does not seem to accomplish quite what was expected of it,” according to Mr. Sawerthal, who recently visited the Observatory at Währing, Vienna. The Director, Dr. Weiss, states in his last report that “the 27-inch Grubb refractor has only been occasionally used, when the objects were too faint for the handier instruments.”
The still larger telescopes erected at the Observatories at Pulkowa and Nice have so recently come into employment that it would be premature to judge of their performance. In the Annual Report from Pulkowa (1887) it is stated that Dr. H. Struve was using the 30-inch refractor “in measuring those of Burnham’s double stars which are only seldom measurable with the ‘old 15-inch,’ together with other stars of which measures are scarce. He made 460 measures in eight or nine months, as well as 166 micro metric observations of the fainter satellites of Saturn and 15 of that of Neptune.” At Nice the 30-inch refractor was employed by M. Perrotin in physical observations of Mars in May and June 1888. The canal-shaped markings of Schiaparelli were confirmed, and some of them were traced “from the ocean of the southern hemisphere right across both continents and seas up to the north polar ice-cap.” The 30-inch also showed some remarkable changes in the markings; but these were not confirmed at other observatories. The telescope evidently revealed a considerable amount of detail on this planet; whence we may infer that its defining power is highly satisfactory.
The great Lick refractor, which appears to have been “first directed to the heavens from its permanent home on Mount Hamilton on the evening of January 3, 1888,” has been found ample work by the zealous astronomers who have it in charge. Prof. Holden, in speaking of it, says:—“It needs peculiar conditions, but when all the conditions are favourable its performance is superb.” Mr. Keeler, one of the observers, writes that, on January 7, 1888, when Saturn was examined, “he not only shone with the brilliancy due to the great size of the objective, but the minutest details of his surface were visible with wonderful distinctness. The outlines of the rings were very sharply defined with a power of 1000.” Mr. Keeler adds:—“According to my experience, there is a direct gain in power with increase of aperture. The 12-inch equatoreal brings to view objects entirely beyond the reach of the 6½-inch telescope, and details almost beyond perception with the 12-inch are visible at a glance with the 36-inch equatoreal. The great telescope is equal in defining power to the smaller ones.” This is no small praise, and it must have been extremely gratifying, not only to those who were immediately associated with the construction of the telescope, but to astronomers everywhere who were hoping to hear a satisfactory report. In its practical results this instrument has not yet, it is true, given us a discovery of any magnitude. It has disclosed several very small stars in the trapezium of the Orion nebula, some difficult double stars have been found and measured, and some interesting work has been done on the planets and nebulæ. Physical details have been observed in the ring nebula, between β and γ Lyræ, which no other telescope has ever reached before.
Mr. Common’s 5-foot reflector has been employed on several objects. In the spring of 1889 Uranus was frequently observed with it, and several minute points of light, suspected to be new satellites, were picked up. Evidence was obtained of a new satellite between Titania and Umbriel; but bad weather and haze, combined with the low altitude of Uranus, interfered with the complete success of the observations. “With only moderate powers, Uranus does not show a perfectly sharp disk. No markings are visible on it, and nothing like a ring has been seen round it.” Mr. Common, in a letter to the writer, dated November 9, 1889, says:—“The 5-foot has only been tried in an unfinished state as yet, the mirror not being quite finished when put into the tube last year. This was in order to gain experience and save the season. It performed much better than I had hoped, and is greatly superior to the 3-foot. I took some very fine photographs with it last year. It has been refigured, or rather completed, this summer, and has just been resilvered.” From this it is evident that Mr. Common’s large instrument has not yet been fully tested; but it clearly gives promise of successful results, and encourages the hope that it will exert an influence on the progress of astronomy. Owing to the highly reflective quality of silvered glass, the 5-foot speculum has a far greater command of light (space-penetrating power) than the great objective mounted at the Lick Observatory. Mr. Common’s mirror may therefore be expected to grasp nebulæ, stars, satellites, and comets which are of the last degree of faintness and quite invisible in the Lick refractor. But we must not forget that the latter instrument is certainly placed in a better atmosphere, and that its action is not therefore arrested in nearly the same degree by haze and undulations of the air. With equal conditions, the great reflector at Ealing would probably far surpass the large refractor we have referred to, the latter having less than one third of the light-grasping power of the former.
This rapid