=========+===================+==============
| FIELD |
DIAMETER +-------------------+ WOLF’S FIELD
| I | III | VII | IN PERSEUS
---------+-----+------+------+--------------
5″ | | | | 3
10 | 7 | 38 | 5 | 34
15 | 27 | 67 | 12 | 52
20 | 25 | 42 | 18 | 26
25 | 6 | 19 | 10 | 6
30 | 5 | 9 | 4 | 2
35 | 3 | 2 | 4 | 1
40 | 4 | 5 | 4 | 2
45 | 1 | 3 | 3 |
50 | 1 | | 3 |
60 | | | 3 |
90 | | | 1 |
---------+-----+------+------+--------------
Exposures of two hours gave all of the nebulae recorded. Five-hour
exposures brightened the images somewhat, but revealed no new ones. The
conclusion would seem to be that the limits of the clusters had been
reached. This, however, is uncertain, for the longer exposures gave
relatively few stars which were not on the plates of shorter exposures.
As a matter of fact, the 24-inch reflector at this altitude seems to
have a maximum working efficiency at slightly over two hours. Save
for very exceptionally clear and steady skies, the longer exposures
add much labor for negligibly greater results. The diameters of the
faintest stars near the center of the field, with the full aperture,
fine sky, and an hour’s exposure, which are of about magnitude 17.5,
are about 2.0″. Longer exposures are apparently subject to change of
focus, differential refraction, and other disturbances which tend to
increase the size of the images unduly, and hence to spread the total
light over a larger area. The result is that the value of _p_ in the
reciprocity equation _Itᵖ_ = _iTᵖ_ does not remain constant throughout
the exposure, but varies, beyond a certain value of _T_, depending on
the adjustments of the telescope, the position of the field, and the
condition of the sky. However, the effect should be more noticeable on
the stars than on nebulae which present surfaces.
[Illustration: FIG. 1.--Distribution of size in Wolf’s Nebel Listen
Nos. 3 to 14]
I have plotted Wolf’s lists of nebulae, Nos. 3-14, in the same manner,
converting estimates of size into seconds of arc according to his
table. These lists were made from plates taken with the 16-inch (41 cm)
Bruce camera, of focal length 203 cm, of the Heidelberg Observatory.
The curves (Fig. 1) take the same form, save that for most the maximum
frequency is for diameters between 20″ and 25″. One of his lists was
made from plates taken with the 30-inch reflector at Königstuhl. It
is of a field in Perseus, α = 3ᵸ12ᵐ, δ = +41° 6′. Diameters of the
124 Wolf nebulae and five others were measured from plates taken with
the Yerkes 24-inch reflector. This plot (Fig. 2) gives a maximum for
diameters around 15″, and the longer focus of Wolf’s 30-inch apparently
does not add to the number of small nebulae distinguishable on the
plates made with the shorter telescope.