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February, 192 3
43
THE NORTH AMERICAN NEBULA IN CYGNUS
Photographed with the Bruce telescope, by E. E. Barnard, with an exposure of 4 hours
This enormous object is composed of self-luminous gases, chiefly hydrogen, helium and nebulum, the last-named of which has been found only in nebulae and not in the sun or on the earth. The form of the nebula cannot be detected by the eye through any telescope, however great — which illustrates the advantages of photography in the study of such objects. The distance may be conservatively estimated at not less than 500 light years, and if this is correct the actual distance across the nebula would be a billion times the scale of our continent.
Visualized Astronomy
Edwin B. Frost Director, Y erh.es Observatory, University of Chicago
NO branch of science has received greater benefit from the art of photography than has astronomy. This has been true particularly since the introduction of the dry plate about forty years ago. A few astronomical photographs had been made with the wet-plate process by the Bonds, father and son, successive directors of the Harvard Observatory, by Louis M. Rutherfurd of New York, by Draper, and by some others. Difficulties in operating with wet plates in the dome of an observatory were, however, too great for any regular or extensive use of photography until the manufacture of dry plates was begun. These are so uniformly good as to quite eliminate the uncertainties at
tendant on the use of wet plates, which might dry out during an exposure or even freeze in winter.
Beginnings of Celestial Photography
Although impressions of the Comet of 1881 had been obtained by Draper and Tanssen, the astronomers have to thank the brilliant comet of 1882 for introducing them to the simplicity and certainty of celestial photography through the dry plate. This was a southern comet, and Dr. David Gill, Director of the famous observatory at the Cape of Good Hope, invoked the aid of a local photographer to secure pictures of the interesting visitor.
Now, it happens that a portrait lens, such as was then chiefly used by photographers in their studios,
is one of the best types for astronomical work on an object like a comet. The camera was strapped to the tube of the telescope and the comet's motions were accurately followed through the telescope by constant use of the slow motion. The result was surprisingly good, and the experiment gave astronomy a new method which has proved to be of incalculable value. Before long it was not safe for a comet to appear in the sky without the certainty that its movements would be closely followed with the camera and all of its freakish changes in appearance made a matter of permanent record on the negative, from which copies practically as perfect as the original could be supplied in indefinite numbers.