On the dispersive Power of the Atmosphere, &s'c. 39 
and the part common to both circles of a reddish yellow, soften- 
ing the colours into each other as they appeared in the planet. 
For I considered that, in fact, it was not a single image of Mars 
that was seen, but a number of differently coloured images, 
lying in the same direction, though lifted one above another, as 
represented in the annexed figures. 
= 
Fig os 
me 
Having prepared a number of these drawings, I repeatedly 
compared them with the planet viewed through the telescope 
with different magnifying powers, carefully noting which figure 
he most resembled, and the time of observation. 
This being done, it was easy to calculate the exact altitude 
from the time of observation, and to make a very near estimate 
of the separation of the images from the figure referred to, com~- 
pared with the diameter of the planet found by the micrometer. 
From a great number of observations on Mars, Venus, and 
the fixed stars, taken in all these different ways, I found the de- 
viation of the extreme rays of light to be between gisth and th 
art of the total refraction. 
It has already been observed, that the disagreement which is 
found to exist between the latitude of a place deduced from ob- 
servations of circumpolar stars, and that from observations of 
the sun, may perhaps be traced to the use of dark glasses. But 
this will appear more evidently from a reference to the method 
employed by Dr. Bradley for determining the quantity of re- 
fraction, which method is very clearly. described by Dr. Maske- 
lyne in the seventy-seventh volume of the Philosophical Trans- 
actions. He says, ’ . 
That Dr. Bradley got the height of the pole from observations 
of the circumpolar stars, and the height of the equator from 
observations of the sun at the two equinoxes ; that he found 
these two altitudes together amounted to 89°, 58’, 3”, which 
being subtracted from 90°, leaves 1’, 57”, for the sum of the 
refractions at the pole and equator ; and that of this quantity 
he assigned 454” to the former, and 712” to the latter. 
But Dr. Bradley undoubtedly made use of dark glasses for 
observing the sun, probably smoked glasses, which would ewe 
C4 im 
