My intention, in the pages following, is to convey in brief, and I hope clear form, my views concerning some of the results already obtained by the aid of photography in the elucidation of celestial problems, the complete solution of which cannot for many years yet be obtained; and I may here quote from the preface to the volume, issued in the year 1893, of A Selection of Photographs of Stars, Star-Clusters, and Nebulæ, the following paragraphs, which are applicable also to the present volume.

“It has been my aim, in publishing the photographs and descriptive matter introduced in the following pages, to place data in the hands of astronomers, for the study of astronomical phenomena, which have been obtained by the aid of mechanical, manipulative, and chemical processes of the highest order at present attainable; and that such data should be, as regards the photographs, free from all personal errors.”

“The photographs portray portions of the Starry Heavens in a form at all times available for study, and identically as they appear to an observer aided by a powerful telescope and clear sky for observing.”

In the processes employed for obtaining the photographic illustrations contained in this volume the same instruments have been used, and the same care has been exercised in the production of the illustrations of the various objects as in the first volume; but owing to improvements in the manufacture of photographic films, and to the extended data now available beyond that which had been obtained up to the year 1893, when the first volume was published, certain deductions concerning the evolution of stellar systems are now permissible which six years ago would have been justly considered premature.