ASTRONOMY: SHAPLEY AND SHAPLEY 
455 
The Constance of light at minimum and the evident variation at maxi- 
mum leads to the conclusion that the minimum phase is the normal 
condition of the star, and suggests that the maxima represent fairly 
equally spaced but otherwise irregular and varied Hght disturbances. 
As to whether the photographic or the visual range is the greater 
(the ratio of the two may not be constant), the data seem to indicate 
that the former is at least equal to the latter, if it does not, in fact, 
considerably exceed it, so that XX Cygni probably is not materially 
different in the matter of color change from other variables of the 
cluster or Cepheid type. 
Phase 
3h oh ih 
11.0 
11.2 
11.6 
11.8 
r 
-h 
/ 
/ 
/ 
^ / 
C L 
; 
; 
/ 
/ 
. / 
/' 
GUTHNICK'S VISUAL LIGHT CURVES OF XX CYGNI FOR SEPTEMBER 22, AND OCTOBER 5 
(BROKEN LINE), 1908, SHOWING TWO EXTREME TYPES OF MAXIMUM. 
Summary. (1) The cause of light variation has not been ascertained 
definitely for the variable stars known as Cepheids or short period 
variables, but in preference to the hypothesis that they are double 
stars an alternative explanation is proposed that seems to harmonize 
much better with observed data. (2) The new hypothesis would as- 
cribe the periodic light and spectrum variations to vibrations in isolated 
stellar masses. (3) The present study of the variable XX Cygni is 
based upon 300 Mount Wilson photographic observations in which the 
variation for light of different wave-lengths is considered, and upon the 
analysis of 2700 visual measures by European astronomers. (4) It 
contributes to the solution of the problem by showing that a supposed 
exception to the normal color phenomena of Cepheids probably does 
not exist. (5) Of more importance to the Cepheid interpretation it 
