Transcriber’s Notes: Underscores “_” before and after a word or phrase indicate _italics_ in the original text. Equal signs “=” before and after a word or phrase indicate =bold= in the original text. Illustrations and tables have been moved so they do not break up paragraphs. Small capitals have been converted to SOLID capitals. Typographical and punctuation errors have been silently corrected. PHOTOGRAPHIC INVESTIGATIONS OF FAINT NEBULAE THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS THE BAKER & TAYLOR COMPANY NEW YORK THE CAMBRIDGE UNIVERSITY PRESS LONDON THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO, FUKUOKA, BOMBAY THE MISSION BOOK COMPANY SHANGHAI The University of Chicago PUBLICATIONS OF THE YERKES OBSERVATORY VOLUME IV PART II PHOTOGRAPHIC INVESTIGATIONS OF FAINT NEBULAE BY EDWIN P. HUBBLE [Illustration] THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS COPYRIGHT 1920 BY THE UNIVERSITY OF CHICAGO All Rights Reserved Published January 1920 Composed and Printed By The University of Chicago Press Chicago, Illinois. U.S.A. PHOTOGRAPHIC INVESTIGATIONS OF FAINT NEBULAE[1] BY EDWIN P. HUBBLE The study of nebulae is essentially a photographic problem for cameras of wide angle and reflectors of large focal ratio. The photographic plate presents a definite and permanent record beside which visual observations lose most of their significance. Perhaps the one field left for the older method is the measurement of sharp nuclei deeply enshrouded in nebulosity. New nebulae are now but rarely seen in the sky, although an hour’s exposure made at random with a large reflector has more than an even chance of adding several small faint objects to the rapidly growing list of those already known. About 17,000 have already been catalogued, and the estimates of those within reach of existing instruments, based on the ratio of those previously known to those new in various fields, lie around 150,000. [1] A dissertation submitted to the Faculty of the Ogden Graduate School of Science of the University of Chicago in candidacy for the degree of Doctor of Philosophy.