The most homogeneous list of nebulae for statistical study is that
compiled by Hardcastle[13] containing all nebulae found on the
Franklin-Adams charts. These are uniform exposures of two hours on
fast plates made with a Cooke astrographic lens of 10-inch aperture
and 45-inch focal length. The scale is 1 mm = 3′. The entire sky is
covered, but since the plates are centered about 15° apart and the
definition decreases very appreciably with distance from the optical
axis, the material is not strictly homogeneous. Moreover, the published
list suffers from the usual errors attendant on routine cataloguing;
for instance, four conspicuous Messier nebulae, M 60, M 87, M 94,
and M 101, are missing. In general, however, the list is thoroughly
representative down to about the thirteenth photographic magnitude and
very few conspicuous objects are overlooked. It plays the role of a
standard with which other catalogues of the brighter nebulae may be
compared for completeness, and numbers in limited areas may be extended
to the entire sky.
When known galactic nebulae, clusters, and the objects in the
Magellanic Clouds are weeded out, the remaining 700 nebulae may
be treated as extra-galactic. Very few can be classified from the
Franklin-Adams plates; for this purpose photographs on a much larger
scale are required. Until further data on the individual objects
are available, Hardcastle’s list can be used only for the study of
distribution over the sky. This shows the well-known features—the
greater density in the northern galactic hemisphere, the concentration
in Virgo, and the restriction of the very large nebulae to the southern
galactic hemisphere.
Fortunately, numerical data do exist in the form of total visual
magnitudes for many of the nebulae in the northern sky. These
determinations were made by Holetschek,[14] who attempted to observe
all nebulae within reach of his 6-inch refractor. He later restricted
his program; but the final list is reasonably complete for the more
conspicuous nebulae north of declination –10°, and is representative
down to visual magnitude about 12.5. Out of 417 extra-galactic nebulae
in Holetschek’s list, 408 are north of –10°, as compared with 400
in Hardcastle’s. The two lists agree very well for the brighter
objects, but diverge more and more with decreasing luminosity. At the
twelfth magnitude about half of Holetschek’s nebulae are included by
Hardcastle. Since the two lists compare favorably in completeness over
so large a region of the sky, Holetschek’s may be chosen as the basis
for a statistical study and advantage taken of the valuable numerical
data on total luminosities.
Hopmann[15] has revised the scale of magnitudes by photometric measures
of the comparison stars used by Holetschek. New magnitudes were thus
obtained for 85 individual nebulae and from these were derived mean
correction tables applicable to the entire list. The revised magnitudes
are used throughout the following discussion. Hopmann’s corrections
extend to about 12.0 mag., and have been extrapolated on the assumption
that they are constant for the fainter magnitudes. The errors involved
are unimportant in view of selective effects which must be present
among the observed objects near the limit of visibility.
The nebulae were classified and their diameters measured from
photographs of about 300 of them taken with the 60-inch and 100-inch
reflectors at Mount Wilson. Most of the others are included in the
great collection of nebular photographs at Mount Hamilton, which
have been described by Curtis;[16] and, through the courtesy of the
Director of the Lick Observatory, it has been possible to confirm
the classification inferred from the published description by actual
inspection of the original negatives.
Types, diameters, and total visual magnitudes are thus available for
some 400 of the nebulae in Holetschek’s list. The few unclassified
objects are all fainter than 12.5 mag. The data are listed in Tables
I–IV, in which the N.G.C. numbers, the total magnitudes, and the
logarithms of the maximum diameters in minutes of arc are given for
each type separately. A summary is given in Table V, in which the
relative frequencies and the mean magnitudes of the various types will
be found.