544 
Proceedings of the Royal Society 
Heriot Row, at the head of Dundas Street, the highest station in 
my walk. As I went westward along Heriot Row the breaking-up 
appeared greater and greater at every interruption of the street, 
which gave me a view of the northern sky ; and when I reached 
the landing-place of my house, in Moray Place, from which, however, 
only the western half of the region of the bow could he seen, the 
whole appearances had vanished, and the sky was everywhere 
mottled with thin grey fleecy clouds, small and of irregular ill- 
defined outline. I did not again look for it, hut I understand it 
was partly seen by others so late as two p.m. 
Various particulars, which it is unnecessary to mention, led me 
to suppose at the time that this colourless how had some connection 
with the smoky column of air through which the sun’s rays pene- 
trated. But this supposition was put an end to by learning from 
my son that, when at Craigiehall, five miles west from town, in a 
smokeless atmosphere, he observed the how distinctly about one 
o’clock. Its edges were never sharply defined so long as he noticed 
it. But it had no colour. Another gentleman present thought 
there was a limited blueness at one place. But my son satisfied 
himself that this was owing to a patch of blue sky behind, and he 
is sure that there was no colour at any part of the how visible to 
him. 
A better explanation has been suggested to me by Professor Tait, 
to whose theory I subscribe. But I leave it to himself to explain 
his views. 
By Professor Tait. 
I was unfortunate in not seeing the phenomenon till nearly 2 p.m., 
when I was on my way from College to the Observatory. It was 
then very faint, hut I saw at once that it differed in a marked 
manner from an ordinary rainbow. Prom what I could see, I attri- 
buted its apparent whiteness to the greatly increased effective surface 
from which tho light came. This was probably due to reflection 
from ice-crystals mixed with the drops of water in the thin strata of 
cloud which covered the whole sky. The sun’s light was much 
dimmed, and the edge of its disc was very indistinct, as the clouds 
immediately round it, to the distance of at least a diameter, appeared 
nearly as bright as the disc itself. Hence this rainbow was probably 
very much less pure, while also much less bright, than the usual one. 
