PHOTOGRAPHIC INVESTIGATIONS
OF FAINT NEBULAE
BY
EDWIN P. HUBBLE
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.