38 On the dispersive Power of the Almosphere, &c. 
The first instrument employed for the purpose with any de- 
gree of satisfaction, was the two-feet reflector made by Mr. 
Short, and which belongs to the Royal Society. In the com- 
pound focus of the eye-piece of this telescope, I fixed horizon- 
tally a narrow slip of ivory. With the instrument thus pre- 
pared, I observed Capella, “and other low stars near the meri- 
dian. By carefully noticing the intervals of time between the 
first contact and total immersion, and between the first appear- 
ance and complete emersion of the star from behind the slip of 
ivory, I obtained data from which it was easy to calculate its 
. vertical breadth, which, compared by estimation with its hori- 
zontal breadth, gave the separation of the extreme rays of light. 
It was impossible, however, to remain long satisfied with 
such coarse measures, and not finding it convenient to go to 
much expense on this account, I applied to my friend Mr. 
Rennie for the loan of his seven-feet reflector made by Dr. 
Herschel, to which I adapted a very excellent wire micrometer 
made by Mr. Troughton ; and thus, by the kind assistance of 
my friends, [ obtained instruments capable of measuring small 
angles to the fraction of a second of space. 
With this apparatus I repeatedly measured the diameter of 
Mars during his opposition in 1813, The Society’s apartments 
being well situated for the purpose, 1 observed the planet as 
soon as he became visible over the buildings, until he attained 
his meridian altitude, which never exceeded 15°. 
Witii a power of 170 and upwards, the disk of the planet ap- 
peared much elongated, especially when near the horizon; the 
upper limb was of a fine blue, the lower limb of a deep red. 
By carefully measuring the diameter of Mars and the breadth 
of the coloured edges, I endeavoured to ascertain, as exactly as 
possible, the degree of separation of the differently coloured 
images of the planet. 
But after all it was no easy matter to measure the coloured 
edges exactly, for the light which was necessary to illuminate 
the wires, seadeted the colours so very faint as to make it ex- 
tremely difficult to distinguish their precise boundaries. For 
this reason, and because I wished to apply higher powers than 
could be used with the micrometer, I adopted the following , 
method, which I found far more convenient, and is, I believe, 
quite as accurate. 
I drew on a sheet of paper several figures of two equal circles 
cutting each other, placing the centres of the circles in the first 
figure (Plate I.) +!,th of their radius from each other; in the se- 
cond figure ;8,ths; in the third ;3,ths; and so on. The upper 
crescent of these figures I painted blue, the lower crescent red, 
and 
