CHANGES OF GEOLOGICAL CLIMATE 503 
1. Barnard, Wolf, and others have shown the very common 
existence of extensive, diffuse, irregular, luminous and non-luminous 
nebulosities in the sky, and I have previously called attention to 
the apparent affiliation of Barnard’s dark nebulae with our local 
cloud of stars; Hubble’s unpublished observations appear to sub- 
stantiate this affiliation of the dark markings and demonstrate 
their relative proximity to the solar system. 
2. The great Orion nebula is one of the luminous or partially 
luminous members of this group of nebulosities, and spectroscopic 
observations by Fabry, Frost, and Campbell and their associates 
have shown that its various parts are moving relative to each other 
with velocities of several kilometers a second. 
3. In and near the Orion nebula more than seventy faint 
stars that vary in light have been found at the Harvard, Heidel- 
berg, and Yerkes observatories, and elsewhere; Lampland has also 
found similar variables in the comparable nebula Messier 8. My 
work on the variables in Orion has shown: (a) no certain perio- 
dicity in the variation, (0b) light curves not comparable with known 
types, (c) a variety of color types among the variables, and (d) no 
evidence of great range or of extinction (as would be expected from 
occultations). 
4. Van Maanen’s discussion of the proper motions supports 
the inference, based on the distribution of the variables, that they 
are really associated with the Orion nebula and with the cluster of 
brighter stars in that vicinity. The distance of this group of stars, 
according to Russell and Kapteyn, is six hundred light-years. The 
diameter of the region throughout which these peculiar variables 
are known is fifty thousand times the diameter of Neptune’s orbit, 
and the total nebulous region in Orion is many times larger. (From 
the present direction and amount of their motions, we compute 
that a few million years ago our sun was in the vicinity of the 
Orion nebula; at its present speed the sun would require nearly 
a million years to pass through that particular nebulous region). 
5. From the known distance of the variables in Orion, and my 
measures of their apparent magnitudes, it is easily computed that 
in luminosity they are dwarfs. This supports the conclusion from 
the study of their light curves that the Orion variables differ 
