350 
Proceedings of Poyal Society of Edinburgh. [sess. 
Note on Dew Bows. By C. G. Knott, D.Sc., and 
R. A. Lundie, M.A., B.Sc., M.B., C.M. 
(Read December 19, 1898. ) 
A dew bow differs from a fog bow in being formed by minute 
globules of water resting on the ground ; but these globules may or 
may not have been produced by the usual process of dew formation. 
In the Proceedings of the Society (vol. vii., 1870), Clerk Maxwell 
has published a note on a bow seen on ice. With the exception 
of this record, we have been unable to find any distinct account of 
observations of the phenomenon, although no doubt it must have 
been occasionally observed soon after sunrise. 
The peculiarity of the bows we observed was that they were 
seen by night, the sources of light being the gas lamps and electric 
lights of the city. It was on the evening of Friday, November 
11, after several days of thick foggy weather. The tiny fog 
particles seem to have gradually settled down in the still air, the 
fog steadily clearing the whole time. These globules, in spite of 
their contact with the damp ground, must have retained their 
spherical form almost perfectly ; for they were able to throw back 
to the eye, with a rainbow deviation of approximately 42°, the light 
incident upon them. 
In the neighbourhood of every lamp where the surface of the 
ground had been undisturbed by traffic the bows were seen as 
bright streaks, which shifted position with the observer. Any 
disturbance of the surface, a wheelmark, a footprint, a finger 
drawn across the pavement, obliterated the bright streak ; and it 
was not seen on the surface of puddles. On stooping, we saw the 
bow run from us along the ground ; and as we moved, it moved 
with us. With the gas lamp as a source of light the streak was 
of one colour ; but with the more powerful electric light as the 
source it was possible to distinguish rainbow tints. 
Every drop of water that is efficient in producing the bow must 
be the vertex of a triangle, whose base is the line joining the 
source of light and the eye of the observer, and whose vertical 
