ASTRONOMY: SHAPLEY AND NICHOLSON 
419 
lines of a pulsating star. From the eclipsing stars that most resemble 
the typical Cepheid variable stars in density, spectral type, and abso- 
lute luminosity, there is evidence in certain cases that the light dimin- 
ishes more rapidly toward the limb than required by the above law. 
On the other hand, however, the relative intensity in the violet region 
of the continuous spectrum of Cepheids and other giant F-type stars 
v-o v=i 
FIG. 2 
may be urged against the assumption of excessive darkening. In any 
case, in view of the existence of large numbers of strong absorption 
lines throughout the spectra, it may be accepted that Cepheid variables 
are considerably darkened at the limb. For them the coefficient x may 
be reasonably assumed to lie between one-half and unity. 
The preliminary assumption that spectral lines are inherently infini- 
tesimal in width is of course untrue. Even if the radiation concerned 
