[ 36 
1X. On the dispersive Power of the Almosphere; and its Effect 
on astronomical Observations. By StEPHEN LE, Clerk and 
Librarian to the Reyal Society. Communicated ly W.H. 
Wot tasron, M.D. Sec. R.S.* 
Norwitnstanprxe the pains which astronomers have taken 
to determine accurately the refraction of mixed light, nothing, 
I believe, has ever been done towards ascertaining the dispersive 
power of common air, or comparative degree of refrangibility 
of the differently coloured rays in their passage through our at- 
mosphere. 
The importance of such an inquiry, however, must be obvious 
to every one who duly considers the effect which the different 
degrees of refrangibility of the variously coloured lights must 
necessarily produce in the apparent situations of differently-co- 
loured objects. Stars of different colours must be differently re- 
fracted, aud the apparent altitude of the sun must vary accord- 
ing to the colour of the dark glass through which he is viewed. 
Perhaps this cause alone is sufficient to explain the disagree- 
ment which is found to exist between the latitude of a place de- 
duced from observations of circumpolar stars, and that deduced 
from observations of the sun during the solstices, which has so 
long occupied the attention of astronomers, and has never yet 
been satisfactorily accounted fort. 
The dispersive power of the atmosphere will also show why 
Aldebaran and the red stars are sometimes seen projected on 
the moon’s disk in occultations by that planet, especially when 
the immersion or emersion happens to be near her upper limb. 
For the light of the moon being white, is more refracted than 
that of the star, and consequently her limb more elevated, which 
would occasion the star to appear within her disk a few seconds 
before or after contact}. 
The great disagreement which is found to exist in the decli- 
nation of several of the fixed stars, as given by different ob- 
servers, may probably be traced to the same cause, stars being 
more or less refracted according to the predominant colour of 
which their light is composed. 
That the fixed stars differ from each other in respect to the 
composition of their light, must be obvious to any one who will 
* From the Philosophical Transactions for 1815, part ii. 
+ Vide Mr. Piazzi’s Memoir on the Obliquity of the Ecliptic, in the Me- 
moirs of the Sucietd Italiana, vol. xi. ‘ 
{ Vide Philosophical Transactions, vol. Ixxxiv. p. 845. Histoire Céleste 
Frangoise, tom. 1. p. 93, 403, 418, 425, 428, 467, and Connoissance des 
Lemps for 1817. 
only 
