A limit is placed on our knowledge of astrophysics which, up to the present time, we have found no means of overstepping.
It has thus come about that astrophysics owes its recent development, and its recognition as a distinct branch of astronomical science, to the combination of the processes involved in the three arts of spectroscopy, photography and photometry.
From 1872 he held the chair of astrophysics at Leipzig University.
From 1892 to 1895 he was an editor of Astronomy and Astrophysics and thereafter of The Astrophysical Journal.
He employed in his discussion the radial velocities of 280 stars, spectroscopically determined; and the upshot signally exemplified the community of interests between the rising science of astrophysics and the ancient science of astrometry.
The subject of Astrophysics does not admit of so definite a subdivision as that of Astrometry.