of the atmosphere, &c. 
379 
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, rendered the colours so very faint as to make it 
extremely 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 T *o °f their radius from each other ; in the 
second figure in the third T 3 ^; and so on. The upper 
crescent of these figures I painted blue, the lower crescent 
red, and the part common to both circles of a reddish yellow, 
softening 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. 
i 
2 
3 
3 C 2 
