154 
HAGEN. 
3. The physical explanation of these light variations is still 
in its infancy. That these light variations are due, not to 
chemical action, but to mechanical motion, has always been 
considered probable, especially in the stars of the Algol type, 
whose light variations are not real but apparent and are 
due to stellar eclipses. In regard to the real variables, we 
had the hypothesis of Klinkerfuss, that the variations of 
brightness are due to tidal fluctuations, caused by the ap¬ 
proach of satellites.* But it was only quite lately that vari¬ 
able stars have been proved to be subject to periodic motions 
in space, and that their light curves and velocity curves are 
synchronous. This has been proved by the spectroscopic 
observation of their velocities in the case of the following 
three stars : 
<5 Cephei, period of light and velocity = 5 d .3, variation in 
brightness : 3.7—4.9, excentricity 0.46. 
f] Aquilse, period of light and velocity = 7.18, variation in 
brightness: 3.5 — 4.7, excentricity 0.47. 
C Geminorum, period of light and velocity = 10.15, varia¬ 
tion in brightness : 3.7—4.5, excentricity 0.22. 
The latter star is discussed by Campbell in the Astrophy- 
sical Journal, vol. 13, p. 90. If the periodicity in the shift¬ 
ing of the spectral lines is due to orbital motion, then the star 
has only one minimum and one maximum of brightness in 
each revolution, and the minimum brightness occurs soon 
after the maximum velocity. The other two variables, on 
the contrary, reach their maximum brightness shortly after 
their maximum velocity. From the changes in velocity the 
excentricities of the orbits have been computed, as noted 
above, and several hypotheses have been advanced as to 
how these elliptical motions could affect the photospheres 
* A particular hypothesis was mentioned in last year’s report on Re¬ 
cent Progress in Astrophysics, which applies to variations of about one 
magnitude (see Bulletin, vol. XIV, pp. 1-20). I take this opportunity to 
correct an inaccuracy on page 12, line 11 from above, where instead of 
“ multiplied by ” it should read “ applied to.” 
