1885.] 
Variable Stars. 
37 
Algol when at his brightest is shining with a light which 
call 5. At his extreme of loss of light he shines with a 
brilliancy which may be represented by 1. Then, ipso facto, 
he has lost four-fifths of his photometric effectiveness. Now 
a planet to cut off this amount of light must, supposing (as 
we may) that it is utterly opaque, have a diameter which 
would bear to the diameter of Algol the proportion of about 
8 to 9. Or, by our assumption as to the mass and brilliancy 
of Algol, — i.e., that it equals the Sun, — the planet must 
have a diameter of nearly 770,000 miles. These figures 
would, perhaps, require some modification for a thoroughly 
exaCt computation, since, in all probability, the greater 
portion of the light of Algol, as in the case of the Sun, 
comes from the centre of that portion of his disc which faces 
us. I have preferred, however, to leave them untouched, 
because the planet need not necessarily cross the star’s disc 
centrally. 
Taking these figures, then, to be correct enough for our 
present purposes, we arrive at some rather strange conse- 
quences ; — (1) Either these two orbs are formed of different 
materials ; or (2) one has in some way been able to retain 
its heat, while the other has parted with its heat in the 
ordinary course of nature ; or (3) the density of the brighter 
of these is far in excess of that of the opaque body. 
Now let us consider the a priori possibility of these in- 
stances seriatim. We shall, I think, as we proceed, perceive 
how unsafe it will be to put our confidence in a theory which 
is so utterly at variance with everything that analogy 
teaches us. 
With regard to (1) it should be clearly understood that we 
have not the least reason to suppose that two bodies in one 
system should differ in their component elements. In the 
Solar System, from the facSt that the Sun is constituted of 
similar materials to the Earth, we infer that the planets 
possess these materials also. And this view is further 
strengthened by the circumstance that Mars and Venus ex- 
hibit certain indications of air and water upon their surfaces, 
giving a yet closer sign of universal relationship in the 
planetary scheme. Any element such as iron or gold need 
not, of course, exist in the exacft proportion to a planet’s 
mass : but the disproportion, whatever it is, must be very 
small, so that we shall be quite within due bounds in 
assuming all the planets in the Solar System to be formed 
similarly. Most assuredly, therefore, we should not be 
justified, without strong positive evidence, in concluding 
that different laws have been in course of enactment Out 
yonder in the star depths. 
