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ASTRONOMICAL INSTRUMENTS

going away from us, it is longer. Thus, by measuring the positions of its lines in the spectrum, it is possible to determine whether a star is approaching us or moving away from us.

In recent years the studies of the spectra of stars have been made almost entirely by photography. It is found that, as in other cases, the sensitive plates now used in that art will take impressions of objects which the eye cannot see in the telescope. So the astronomer photographs the spectrum of a star, which will show all the lines he can see with the naked eye, and perhaps a great many more. The positions of these lines are measured and studied, and the astronomer's conclusions are drawn from these studies.

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