File:Astronomy for high schools and colleges (1881) (14577197308).jpg

Identifier: astronomyforhigh00newc (find matches)
Title: Astronomy for high schools and colleges
Year: 1881 (1880s)
Authors: Newcomb, Simon, 1835-1909 Holden, Edward Singleton, 1846-1914, joint author
Subjects: Astronomy
Publisher: New York, H. Holt and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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notsatisfactorily show such details. As the granulations of the solarsurface are, in general, not greatly larger than 1 or 2, the photo-graphic irradiation, which is sometimes 20 or more, may completelyobscure their characteristics. This difficulty M. Janssen has over-come by enlarging the image and shortening the time of expos-ure. In this way the irradiation is diminished, because as the di-ameters increase, the linear dimensions of the details are increased,and the imperfections of the sensitive plate have less relative im-portance. THE SUNS PHOTOSPHERE. 281 Again, M. Janssen has noted tlwit in short exposures the photo-graphic spectrum is almost monochromatic. In this way it differs greatly from the visible spectrum, and tothe advantage of the former for this special purpose. The diameterof the solar photograms have since 1874 been successively increasedto 13, 15, 20, and 80 centimetres. The exposure is made equal allover the surface. In summer this exposure for the largest photo-
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FlG. 77.—RETICULATED ARRANGEMENT OF THE PHOTOSPHERE, grams is less than 00005. The development of such pictures isvery slow. These photograms, on examination, show that the solar surface iscovered with a fine granulation. The forms and the dimensions ofthe elementary surfaces are very various. They vary in size from0-3 or 04 to 3 or 4 (200 to 3000 kilometres). Their forma 282 ASTRONOMY. are generally circles or ellipses, but these curves are sometimesgreatly altered. This granulation is apparently spread equally allover the disk. The brilliancy of the points is very variable, andthey appear to be situated at different depths below the photo-sphere : the most luminous particles, those to which the solar lightis chiefly due, occupy only a small fraction of the solar surface. The most remarkable feature, however, is the reticulated ar-rangement of the parts of the photosphere. The photogramsshow that the constitution of the photosphere is not uniformthroughout, but that it is divided i

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