486 
Augustine’s bay on the south-western 
shore, and Tamatave and Antongil 
bay on the eastern, scarcely any part 
is known to English shipping,—the 
French being the only power that re- 
sorted thither till Mauritius came into 
our possession., Since that time we 
have, though very slowly, gained a 
footing; and the king of the eastern 
division of the island is now friendly, 
though sufficiently jealous of the en- 
croaching spirit of Europeans. The 
many attempts made by France to ac- 
quire the entire sovereignty of the 
island are not forgotten. But the 
fierce, warlike, and predatory, spirit of 
the people, gives great facilities for 
the slave-trade to the flesh-dealers of 
Mauritius and Bourbon. A gentle- 
man, lately arrived thence, stated to a 
friend of mine here, that he was pri- 
vately informed of four or five cargoes 
being landed there within a very short 
space of time. / 
—— 
For the Monthly Magazine. 
On the ALTERNATION of the COLOURS of 
certain FIXED STARS; by DR. T. FORS- 
TER, F.L.S. member of the ASTRONO- 
MICAL SOCIETY of LONDON. 
HE alternation of the colours of 
the light of certain stars does 
not appear to me to have sufficiently 
engaged the attention of astronomers ; 
and it is with a view to excite enquiry 
into its cause that 1 am induced to 
offer the following observations. 
Some years ago, on looking towards 
the constellation of the Scorpion, 1 
observed a remarkable changing of 
colour in Antares: for a second or two 
of time it appeared of a deep crimson 
colour, then of a whitish colour; then 
the crimson was resumed, and so on 
at alternating periods. Sometimes 
every other twinkle (if I may be 
allowed the expression,) showed the 
red colour, while the alternating 
twinkle appeared of the ordinary co- 
lour of star-light. 
What is commonly called the twink- 
ling of a star seems to be an apparent 
fit of dilatation and increased briiliancy, 
rapidly succeeded by the opposite 
state of apparent contraction of sur- 
face and dulness. I have observed, 
also, that the twinkles are of longer or 
shorter duration, at different times: 
now, in general, the crimson light I 
allude to occurs in every other dila- 
tation, but sometimes only in every 
third, and at other times quite irregu- 
Dr. Forster on the Twinkling of Stars. 
[Jan. 1, 
larly: moreover, it lasts longer some- 
times than at others, and scarcely 
ever exceeds two seconds of time at 
once. 
I have formerly published accounts 
of this phenomenon in the Journals, 
and have ascribed it to some sort of 
change in the star itself, or to a revo- 
lution round its axis, whereby different 
coloured portions of the sphere are 
presented to us: but this explanation 
vanishes ona moment’s reflection ; and 
I am inclined to ascribe it to some 
atmospherical cause. I have some- 
times thought that the upper portions 
of the atmosphere might haye some 
undulatory motion, and that the alter- 
nating colour might be produced by 
its refractive powers: for the atmos- 
phere, in this case, acting as an im- 
perfect prism, might present different 
colours, according to the varying in- 
clinations of its wavy surface. I have 
thought, too, that portions of the 
aqueous atmosphere, possessing dif- 
ferent refractive powers, might be 
transmitted downwards in dew, or 
that there might be some other un- 
known motion in the real air, which 
might cause the appearance. Antares, 
Betalgeus, Aldebaran, and other red 
stars, show this change of colours 
very strongly, particularly the former ; 
while Syrius, and the light. stars, 
scarcely present any alternation of 
colour. This may in either case be 
owing to the different composition of 
their light, which would materially in- 
fluence the refracted spectrum. Col- 
lateral experiments, and the mere 
appearance of stars in chromatic. te- 
lescopes, tend to prove that the light 
of different celestial bodies is diffe- 
rently composed. 
Some interesting observations on 
the Dispersive Power of the Atmos- 
phere, published a few years ago by 
Mr. Stephen Lee, contain an account 
of the composition of the light of some 
of the principal stars ; and no one can 
reflect on the influence which all the 
above varieties must have on tables of 
refraction, without at once seeing the 
utility of multiplying and correcting 
observations on it. 
Additional observations are. still 
wanting to explain the cause of these 
phenomena; but I shall still be ex- 
cused, I trust, in the absence of more 
matured and extended observations, 
for this imperfect attempt to excite 
the attention of philosophers to facts, 
which 
