380 Mr. Lee on the dispersive power 
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 esti- 
mate of the separation of the images from the figure referred 
to, compared 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 
deviation of the extreme rays of light to be between ^ and 
T 3 part 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 
observations 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 quan- 
tity of refraction, which method is very clearly described by 
Dr. Masrelyne in the seventy-seventh volume of the Philo- 
sophical Transactions. He says. 
That Dr. Bradley got the height of the pole from observ- 
ations 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 i', 57", for the sum 
of the refractions at the pole and equator ; and that of this 
quantity he assigned 45^" to the former* and 71 5" to the 
latter. 
