ASTRONOMY: ADAMS AND SHAPLEY 
137 
they were classed as Cepheids, and, in fact, every light-variable of the 
Cepheid type, when put to the test, has proved to be a velocity-variable 
with the distinct Cepheid peculiarities. Any definite contribution to 
the explanation of the light or velocity variations of a single Cepheid, 
therefore, is of unusual value in that it involves the interpretation of the 
majority of all variable stars. 
The present state of our knowledge of the causes of Cepheid variation 
has been discussed in former papers. The clearest outcome of recent 
studies of the subject is the apparent impossibility of relating the velocity 
variations to orbital motions. Probably the strongest evidence on this 
point is the synchronous variations of the magnitude and the spectral 
class, recently observed at Mount Wilson for two cluster- type Cepheids. 
Such variations of spectrum have received little consideration hereto- 
fore, notwithstanding the known change in color from maximum to 
minimum light that would seemingly demand the changes in spectral 
lines that distinguish one class of spectrum from another. 
The question naturally arises whether in suitable investigations of 
the brightest and most typical Cepheids changes of spectrum would not 
be found analogous to those observed in the less typical variables RS 
Bootis and RR Lyrae. Might it not also be possible with a special 
analysis of high dispersion plates to bring out other characteristics of 
the spectrum that would throw light on the causes underlying the hght 
and velocity variations? With these points in view an investigation of 
the spectrum of 8 Cephei was made, and the results are briefly outlined 
in the following paragraphs. The discovery of the conspicuous change 
of the spectral class of the first and best known Cepheid indicates that 
constancy of spectrum is to be expected for none of this class of varia- 
bles, and, coupled with the remarkable behavior of the spectral lines, 
suggests that the possibility of a completely satisfactory theory of Ce- 
pheid variation is not necessarily remote. 
Two photographs of the spectrum of 8 Cephei were obtained on the 
nights of December 23 and 24, 1915, with the Cassegrain spectrograph 
and 60-inch reflector. The full optical train of three prisms and a 
102 cm. camera was employed. This combination gives a linear scale at 
Hy of 5.3 angstroms to the millimeter. The data for the photographs 
are as follows: 
Plate 7 4571 Dec. 23, G.M.T. 15H'" Exposure time 225"^ 
Plate 7 4578 Dec. 24, G.M.T. 14^7- Exposure time 120- 
The first photograph was taken under exceptionally poor conditions, 
and this fact is responsible for the extended exposure time. As it is, 
