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 comparative light of stars can be studied from photographic plates. The meridian photometer1 is an ingenious device for doing this in the most convenient manner, and the results are surprisingly accurate. Again, photography is an easy means of obtaining excellent charts of the heavens. These photographic telescopes bring into view stars that cannot be seen by the eve, even with the most powerful telescopes. Thus we have a record of every star in the heavens, down to the faintest of which we can be made aware. The positions of the stars cannot be so accurately determined by these charts as by the meridian circle. But they are accurate enough for some purposes and, what is most important, they give us a record of thousands of stars that never could have been observed singly. The plates of the same region taken with the same telescope at different times may be compared and any changes noted. The new star in Auriga, which drew so much attention two years ago, was found to have printed itself upon one of the Harvard plates weeks before its discovery. One of the most important uses of photography is in spectrum analysis. It is by means of the spectroscope that the most important advances in modern astronomy have been made. The rainbow-colored band, crossed by its tale-telling black or bright lines, has let us into the physical secrets of other worlds to a marvellous extent. Here at Harvard the spectra are photographed by placing a prism in front of the object glass of a photographic telescope. Of course the prismatic hues are not reproduced on the plate, but the lines are very distinctly marked, and can be studied at leisure. The most interesting of all the photographic telescopes
1 Erratum.At the top of page 140, omit the sentence beginning “The meridian photometer.” It is true that stellar magnitudes are conveniently and accurately derived from measurements on photographic plates. The meridian photometer, however, is not a photographic instrument, but is used for the visual determination of stellar magnitudes. It is an important part of the equipment of the Observatory.
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