1901 - 2 .] Dr A. W. Roberts on Algol Variation. 
83 
The accordance between them is the measure of the validity 
and reliability of the conclusions arrived at. 
Three equations are sufficient for the determination of the 
theoretical magnitude at any instant : 
In which, 
T = interval in minutes to or from the minimum date 
July 2, 16 h . 17 m . 
L = amount of light at time T 
M = magnitude of star at time T 
With these equations, theoretical values of the magnitude of 
the variable for instants when actual observations were made 
were computed. 
These computed values are given in the following table, column 
(5), and the residuals between the computed and observed 
magnitudes in column (6). The mean observed light curve and 
the corresponding residuals are given in columns (3) and (4). 
It will be seen that the theoretical and observed light curves are 
practically identical. As exhibited in column (7), the divergence 
between the two curves never exceeds one-hundredth of a 
magnitude. In fig. (2) the theoretical light curve is shown as a 
dotted line. 
[I have not been unmindful in the foregoing investigation of 
the possibility of different portions of the stellar discs being un- 
equally bright. I think, however, the assumption that the stars 
are in that stage of development when a distinct atmosphere has 
not yet been formed is reasonable. If no light absorbing outer 
layer exists, then all portions of the stellar discs will be equally 
luminous.] 
(«) 
</ 8 ) 
(?) 
cos </> = 3-077 -0-991 cos 2 (0'135 T)° 
L= 1 - 0-274(2$ - sin 2<j>) 
M = 2-5(10 - Log L) + 10 m '05 . 
[Table. 
