The Variable Star Algol. 345 
diminishing length of period. They might, however, be repre- 
sented by having regard to the third and fourth power of the time; 
or, still better, by introducing a correction to a uniform period, 
which should progress according to sines and cosines in such a 
way that the p** minimum after the epoch # would be given by 
the formula, 
E+ap+bsin (np + B)+esin(2np+C)+..... 
But the endeavor to develop the constants, even taking account 
of the first term only in the series of sines, proves fruitless, owing 
to the insufficiency of the data; of which, indeed, we have a 
tolerable number for the last century, but which since then are 
so scarce, that for the first forty years of the present century I 
am only aware of 19 observed minima. In the last few years 
certainly the attention of astronomers has been again more di- 
tected to this remarkable phenomenon; but if we are soon to ar- 
tive at an accurate knowledge of the phenomenon itself, and of 
the rules according to which the period varies, the number of ob- 
servers must be very considerably increased. Out of the 127 or 128 
minima which occur in a year, there are scarcely 40 for which 
or 8™, For comparison-stars I use ¢ and 9 Perset, and « and 3 
Trianguli. The star 9 Persei indeed is itself somewhat variable, 
but its period is longer, and it is especially favorablee on account 
of its proximity to Algol, and because it is very nearly equal to 
is star when at its minimum. Ina comparison of Algol with 
@, the little variations from uniformity in the transparency of the 
air, which always exist to a greater or less degree, will exert the 
Smallest possible influence. ; 
Concerning the manner in which the comparative brilliancy o 
‘ars is to be observed without the aid of instruments, I have 
Spoken in detail in another place ;* and will here only briefly re- 
— ._ * Schumacher’s Jahrbuch fiir 1844, p. 191 et & 
Szconp Srares, Vol. XIX, No. 57—May, 1855. 44 
