14 
ASTRONOMY: KAPTEYN AND ADAMS 
The fan is on the same circuit with the electric lights that illuminate 
the setting circles of the telescope. As a consequence the fan is in 
operation at frequent intervals throughout the night. As the lights 
must be turned off whenever an exposure is being made, the fan is out 
of operation and cannot cause the tube to vibrate at any time that this 
could be harmful. 
These devices have proved successful in removing the aberrations 
due to temperature. The Hartmann test has since been applied on a 
number of occasions and has invariably indicated an excellent figure 
for the objective. 
It is not to be supposed that the Thaw objective is peculiar in its 
behavior with respect to changes in temperature. An examination of 
the literature on this subject with regard to other objectives reveals the 
fact that some of them, at least, are similarly affected and exhibit aber- 
rations that change from time to time, precisely as the Thaw objective 
did before the application of the ventilating devices. It therefore appears 
to me that the installation of similar arrangements in the case of other 
telescopes might bring about a considerable improvement in their per- 
formance. 
The details of this investigation are to appear in Volume 4 of the 
Publications of this Observatory. 
THE RELATIONS BETWEEN THE PROPER MOTIONS AND THE 
RADIAL VELOCITIES OF THE STARS OF THE SPECTRAL 
TYPES F, G, K, AND M 
By J. C. Kapteyn and W. S. Adams 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION 
OF WASHINGTON 
Presented to the Academy. November 16. 1914 
Among the stars observed for radial velocity with the 60-inch reflector 
of the Mount Wilson Observatory, there are many of the spectral types 
F, G, K, and M which have either very large or very small astronomical 
proper motions. The two classes of stars show some very marked differ- 
ences which indicate a close relationship between proper motion and 
radial velocity. It is the aim of the present note to bring out this re- 
lationship as clearly as possible. 
For this purpose it is necessary to supplement the Mount Wilson data 
for the stars of extreme proper motion with similar data for the rest of 
the stars. There is, of course, no better source for this purpose than 
the catalogue of radial velocities published recently by the Lick Observa- 
