[12] _H_{β}_ is brighter than N_{2}. Patches with similar spectra are often found in the arms of late-type spirals—N.G.C. 253, M 33, M 101. The typical planetary spectrum, where _H_{β}_ is fainter than N_{2}, is found in the rare cases of apparently stellar nuclei of spirals; for instance, in N.G.C. 1068, 4051, and 4151. Here also the emission spectra are localized and do not extend over the nebulae. [13] _Monthly Notices_. ❰74❱, 699, 1914. [14] _Annalen der Wiener Sternwarte_, ❰20❱, 1907. [15] _Astronomische Nachrichten_, ❰214❱, 425, 1921. [16] _Publications of the Lick Observatory_, ❰13❱, 1918. [17] Since C is constant for all nebulae in a given class, the linear relation between Δ log d and C for the different classes is something more than a mere geometrical relation arising from the observed equality of the mean m_{T} in the various classes. [18] This is apparent even among the observed classes. Referring to formula (3), m_{T} + 5 log b will be constant in so far as C_{e} + 5 log (1 – e) is constant. The following table indicates that the latter term is approximately constant throughout the sequence of elliptical nebulae. The values of C_{e} were read from the smooth curve in Fig. 6. ======+=========+================+==========+=========== e | C_{e} | 5 log (1 – e) | C_{0} | Res. ------+---------+----------------+----------+----------- 0 | 10.30 | 0.0 | 10.30 | –0.14 1 | 10.65 | – .23 | 10.42 | – .02 2 | 11.00 | .48 | 10.52 | + .10 3 | 11.35 | 0.78 | 10.57 | + .13 4 | 11.70 | 1.11 | 10.59 | + .15 5 | 12.05 | 1.50 | 10.55 | + .11 6 | 12.40 | 1.99 | 10.41 | – .03 7 | 12.75 | –2.62 | 10.13 | –0.31 +---------+----------------+----------+----------- Mean | | | 10.44 | 0.12 ------+---------+----------------+----------+----------- [19] _Mt. Wilson Contr._, No. 191; _Astrophysical Journal_, ❰52❱, 162, 1920. [20] Pease, _Mt. Wilson Comm._, No. 51; _Proceedings of the National Academy of Sciences_, ❰4❱, 21, 1918. [21] Pease, _Mt. Wilson Comm._, No. 32: _ibid._, ❰2❱, 517, 1916. [22] _Astrophysical Journal_, ❰55❱, 406, 1922. [23] _Monthly Notices_, ❰82❱, 133, 1922. [24] _Astronomical Journal_, ❰28❱, 75, 1914. [25] _Mt. Wilson Contr._, No. 297; _Astrophysical Journal_, ❰62❱, 168, 1925. [26] The latest and most reliable results bearing on the distribution of faint (hence apparently distant) nebulae are found in Seares’s revision and discussion of the counts made by Fath on plates of the Selected Areas with the 60-inch reflector. When the influence of the cluster in Virgo is eliminated the density appears to be roughly uniform for all latitudes greater than about 25°. [27] _Monthly Notices_, ❰78❱, 3, 1917. [28] Haas, _Introduction to Theoretical Physics_, ❰2❱, 373, 1925. Transcriber’s Notes