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 the constellation of Orion agree in possessing a certain dark line, which appears to coincide in position with one of the bright lines in the famous nebula in the same constellation. He remarks that, with the single exception of Algol, he has not observed this same line in any other white star. These observations naturally suggest the remark that the stars in the constellation of Orion possess a certain affinity beyond that implied by their proximity in the same constellation. They are apparently a group associated by community of composition. In considering this circumstance we are reminded how the Great Nebula, with every increase of optical power and every increase in the period of exposure of the photographs, seems to cover an ever-widening area, extending, as we now know, so as to include several of the bright stars.

Still one more application of the spectroscopic method of measuring movements in the line of sight is found in Dunér's beautiful observations of the limb of the sun. By comparison between the approaching edge and the retreating edge he has been able to ascertain the velocity of the sun's rotation. It is not only interesting to find that these results corroborate the determinations already familiar by observations of the sun-spots, but the spectroscopic method admits of being applied to zones in the sun from which spots are absent. We thus obtain a very complete knowledge of the laws of rotation of our luminary. Dunér's measurements confirm the extraordinary fact that the equatorial regions in the sun accomplish a revolution in a shorter time than zones which are nearer to his poles.

The address of course gave some account of the progress of the combined effort to produce a great photographic