C 4°9 1 
XXVIII. On the Astronomical Refractions. By J. Ivory, 
A. M. F. R. S. 
Read June 19, 1823. 
1. It was known to the ancient astronomers that there is a 
difference between the real and apparent places of the stars, 
arising from the refraction of light in its passage through 
the atmosphere. Tycho Brahe' was the first who attempted 
to free his observations from the effect of this irregularity. 
Since his time, the astronomical refraction has become more 
and more an object of attention, as it is found to have the 
greatest influence on the delicate exactness of modern ob- 
servations. In the course of the last twenty years, many 
researches on this subject have been published by philoso- 
phers of the first note, who have applied all the resources, 
both of theory and practice, to overcome the difficulties which 
it presents. By these means our knowledge has been greatly 
extended ; but the problem of the refractions must still be 
considered as the most imperfect part of modern astronomy. 
The first hypothesis for bringing the astronomical refrac- 
tion under a regular mode of calculation was proposed by 
Cassini. He supposed that the atmosphere is a spherical 
shell consisting of a transparent fluid uniform in its density, 
which reaches to a certain height above the earth’s surface. 
In this manner the change in the direction of the light coming 
from a star, is effected at the outer surface of the pellucid 
MDCCCXXIII. 3 G 
