ister to the Netherlands COPYRIGHT, 1902, BY J. B. PURDY HENRY WHITE Formerly Minister to Switzerland and now appointed Min- Formerly Secretary of the Embassy at London and now appointed Ambassador to Italy Here there are four telescopes, including the great Bruce photographic instrument with which Saturn's ninth satellite was discovered. The peak of El Misti, behind, is 15,000 feet high, and a meteorological station has been established on its summit. This observatory is situated on Mount Hamilton, near San José, California, 4,200 feet above sea level. Its two most powerful instruments are the great refractor of 36 inches aperture, and the Crossley reflector used for photography. Situated at Williams Bay, Wisconsin, but belonging to the Chicago University. Its great refractor is 40 inches in aperture, and the tube is 65 feet long. This is the most powerful telescope, at present in operation, in the world. ASTRONOMICAL PHOTOGRAPHY T By Garrett P. Serviss. eye. The heavenly bodies pour forth rays that are not light, as our eyes understand light, but which are capable of making visible impressions on a chemically prepared surface. Thus objects and strange shapes of things in the sky, of whose existence we should never have dreamed if we had been limited to the visual use of telescopes, are rendered visible in photographs. HE triumphs of astronomical pho- tion which make no impression upon the tography have lately followed so close upon one another's heels that it is not easy to keep track of them. Three new satellites have within a short time been added to the known number of members of our solar system, by the aid of photography; while discoveries of new stars, variable stars, double stars too close to be separated with telescopes, star clouds and black gaps and lanes in the Milky Way, nebulæ of wonderful forms entangled in star-clusters, and great spiral and vortical nebulæ, some resembling whirlpools and some spinning pinwheels, are continually being announced. Photography has so revolutionized the methods of astronomical observation that Herschel, Arago, and Bond, returning to their observatories, would be amazed to see, in place of the traditional grayhaired astronomer gazing through his mighty telescope to find new wonders in the heavens, a roomful of young women bending over little plates of glass and, with microscopes and delicate measuring instruments, intently studying minute black specks to discover new stars and satellites among them. For the finer work of the astronomer the eye has been superseded by the sensitized photographic plate, and the modern method consists, not in gazing into the heavens at night, but in letting the heavens picture themselves on photographs which can be studied at leisure in the full light of day. The photographic plate placed in the focus of a telescope has proved itself far superior to the human retina in the power to penetrate celestial space and to reveal the infinite variety of phenomena existing there. Its superiority depends principally upon three things: Second, the photographic plate can accumulate the impressions made by the light-waves impinging upon it, and pile up their effects until, though insensible at first, they become at length unmistakable and even conspicuous. The power of the eye to see reaches its limit in about one-tenth of a second. During that brief interval the image on the retina may strengthen, but it can go no further. There can be no accumulation of the effect, and if the eye is strained in the attempt to see more, its power weakens, the vision becomes blurred, and the final impression is less distinct than the first. The retina is simply like a mirror which reflects just the light that strikes it, but retains nothing, and the moment the light is withdrawn becomes as blank as before. Third, as may be inferred from what has been said above, the photographic plate makes a permanent and original record. What one man's eye alone sees can be shown to another only through the intervention of the hand, imperfectly representing by a drawing the object of vision. But the photograph makes its record on the spot, without error and subject to no idiosyncrasy. You cannot draw a star or a cluster of stars; no artist could truly represent a great nebula; the complicated landscapes of the moon defy all attempts at delineation-but the Photographed by the Crossley reflector; exposure two hours. This assemblage, composed, it is believed, ing from them are translated by the sen- From the fact that the astronomical photograph is a permanent record it results that a picture of a part of the sky may be taken on one side of the earth, and a new star or new planet contained in it may be discovered on the other side when the plate is sent there quarters and its corps of trained inspectors and investigators at Cambridge, near Boston, while its most powerful photographic telescope is in the southern hemisphere, at Arequipa, Peru. The plates exposed at Arequipa are developed, and then, without particular examination, except in special cases, are carefully packed up and forwarded to Cam bridge, where, as fast as it can be done thoroughly, they are minutely inspected for anything new or remarkable that they may contain. But even if nothing new is found upon them at the time, it by no means follows that something new, and possibly startlingly new, may not be there. They are labeled and laid away for reference, because after a while they will be succeeded by new plates representing the same parts of the heavens at a later date, and a comparison of the new plates with the older ones, may reveal at once some remarkable change taking place among the stars. Thus these plates may be likened to the volumes of a great library, or to the files of a daily newspaper. They are truly historical in their nature. They contain a selfwritten history of the starry heavens, such as it would have been utterly impossible to procure, and as could hardly have been dreamed of, a quarter of a century ago, when astronomers had to depend upon eye observations. A real and trustworthy history of the stars became practicable only after the development of astronomical photography. This was recognized nearly twenty years ago, when an astronomical congress assembled in Paris and determined that a photographic chart of the entire heavens should be made. Fourteen observatories in various parts of the world afterward took part in the making of this chart, which consists of a great number of plates, each covering a small space of the sky. But the plan of the Harvard astronomers is different, since their method is to pho |