NATURE 
[ Fuly 22, 1886 
the class of stars in which, if you remember, I told you that the 
variability probably did not depend upon the star itself, but 
upon its surroundings ; and this is the famous star Algol, which 
is always visible in our latitudes. The history of the light changes 
of Algol is this. If we take the beginning of a cycle it is a star 
of the second magnitude. Suddenly in three hours it goes down 
to the fourth, and then it comes up in another three hours to the 
second, and goes on again for very nearly three days ; and then 
it goes down again, comes up again, and goes on again for 
another three days, and so on, The diagram shows the exact 
lel 
Fic. 29.—Light curve of Algol. 
hape of the light curve as 
d it has been determined by Prof. 
Pickering, dividing the light 
into a thousand parts. 
Fic. 30.—Plan and section of the orbit of the Companion of Algol. 
There is another star very like this—a star which is in 81° N. 
declination, No, 25 in a well-known Catalogue. The difference 
| the dark body it eclipses the light body. 
between Algol and this is that the rise and fall are a little more 
rapid. Its light is feeble for about the same time as the other 
one, but at the bottom the curve is flat, by which I mean that, 
instead of going suddenly down and coming suddenly up again, 
| it stops at its least luminosity for some little time. 
Prof. Pickering has shown, I think, beyond all reasonable 
oubt that what is happening is this. If we take this diagram to 
epresent in plan a large star giving out light, and B, Cc, D, 
=, F represent also in plan different positions of a dark body 
( 
1 
I 
| revolving round that central star; and then if you take the 
t 
hing in section, so that the star and its satellite are repre- 
ented as they really are in the plane which joins the earth 
and the star, you will see that in one part of the revolution of 
5 Now, a further inyes- 
tigation of those conditions in the case of the second star has 
shown that there must be a total eclipse, and therefore Prof. 
‘ickering draws the conclusion that in the former case the light 
of the body which revolves round the central one may be con- 
sidered as 27/—that is to say, that it isa dark body ; but that in 
the case of the star D. 25—81° N.—there must be lumin- 
osity from the star which eclipses the other. Anda very beautiful 
justification of that has recently been noted, because, although 
there is no change in the spectrum of Algol, there is a consider- 
able change in the spectrum of that star the bottom curve of which 
is flat, showing that probably the companion has a large coronal 
atmosphere, and that the light of the central star has to pass 
nrough it. The light of the composite star practically changes 
from green to red very much as our sunlight would change if it 
had to pass through the atmosphere of another sun like itself 
coming between us. 
I have prepared two or three other notes with regard to those 
special matters touching the stars which depend upon their 
distances, to show you that our sun, after all, is a small star— 
that there are several suns in the universe near enough to us to 
have had their distances already determined, which are consider- 
ably more brilliant and more imposing in every way than the 
star which is near us. But the clock tells me that I must leave all 
that to some other time, and I now end the course by thanking 
you very much for the indulgence that you have shown me in 
listening to what I have been able to tell you with regard to the 
constitution of our central body, and to the application of the 
knowledge which we have got in that way to an endeavour to 
cull some of the secrets of the physical construction of those 
suns which are very much farther removed from our ken, 
J. NorRMAN LOCKYER 
y, 
