72 Proceedings of Royal Society of Edinburgh. [sess. 
in the line of sight — which produces variation of the Algol 
type. 
That is, every Algol variable is a binary system, the plane of 
whose orbit is coincident, or practically so, with the plane of 
sight ; eclipse, either partial or total, taking place when the stars 
cross the line of sight. It is evident that if both stars are 
luminous, there will be two eclipses each revolution. 
We have just said that the character, extent, and duration of 
either of these two eclipses in any Algol system depend on the 
elements of the orbit, and on the relative size and brightness of 
the component stars. 
If the latter he known, we can determine from simple geometrical 
principles what the amount of eclipse will be. 
In practice, however, the problem is the converse of this : — 
from the form of the light curve, as indicated by observations, we 
seek to determine the elements and dimensions of the orbit. 
Beyond the interest which belongs to phenomena the interpre- 
tation of which we are able to declare, there is this added interest 
connected with all Algol variables — that, intimately associated with 
the problem of their variation, are other problems bearing directly 
on some of the most profoundly interesting matters in astronomical 
science. 
Tor example, a full determination of the elements of any Algol 
binary star yields material for a further determination of the 
density of the system. 
It is not necessary to know the actual dimensions of the system ; 
all we require to know is the relative dimensions in order to 
arrive at this important result. 
Then, again, there are certain Algol stars, the components of 
which revolve round one another in contact. These remarkable 
systems are at that stage of stellar revolution when bipartition is 
on the eve of taking place. 
Only five such systems are at present known; of these, three 
are in the southern sky. 
Now, observations of the light changes of these stars, when 
conducted with the greatest care and precision, and treated as 
rigorously as one would the most refined measures, will reveal, I 
think, the figure of equilibrium which two contiguous stars 
