1885.] 
Variable Stars. 
35 
In like manner Prof. Balfour Stewart’s suggested possi- 
bility must be dismissed as unfounded upon faCt. “ We are 
entitled,” be says, “ to conclude that, in our own system, 
the approach of a planet to the Sun is favourable to lu- 
minosity, and especially in that portion which is next the 
planet. Let us consider variable stars. Now if we suppose 
such a star to have a large planet revolving round it at a 
large distance, then, according to our hypothesis, that portion 
of the star which is near the planet will be more luminous 
than that which is more remote; and this state of things 
will revolve round as the planet itself revolves, presenting 
to a distant spectator an appearance of variation, with a 
period equal to that of the planet. And if we suppose a 
planet to have a very elliptical orbit, then for a long period 
of time it will be at a distance from its primary, while for a 
comparatively short period it will be very near. We should, 
therefore, expeCt a long period of darkness, and a compara- 
tively short one of intense light, precisely what we have in 
temporary stars.” This theory is, of course, possible, albeit 
somewhat deficient in analogous probability; but it must be 
borne in mind that a spectroscopic examination of one of 
these temporary stars has most conclusively proved that its 
sudden accession of brilliancy was caused by an outbreak of 
glowing hydrogen ; and we have yet, I apprehend, to find 
that the periodical increase and decrease in the light of Algol 
is caused by a conflagration of any substance whatever. 
Moreover, it seems somewhat hazardous to maintain that a 
planet’s influence could generate changes of light to such a 
degree, even if it could affeCt it at all, changes which would 
most obviously suggest an excessive influence over the 
physical condition of the whole system of which it formed 
a part. 
Much more worthy of consideration is that theory which 
assumes each star to be the centre of a system of revolving 
worlds which in some instances pass in the course of each 
revolution, between their primary and the earth. We are 
upon safe ground here, for everything leads to the inference 
that other worlds do exist upon which intelligent beings may 
exist to utilise the metals, &c., which we know to have a 
place in their system as they do in ours. 
Now let us, still keeping chiefly to a consideration of the 
star Algol, endeavour to find a theoretical satellite which 
will account satisfactorily for the periodic changes which its 
ruling sun exhibits. We will, in the first place, assume that 
his mass and volume are equal to the Sun’s. Now Algol 
effects his complete cycle of changes in 69*83 hours 
