of Edinburgh, Session 1869 - 70 . 
69 
5. On a Bow seen on the Surface of Ice. By J. Clerk 
Maxwell, Esq., E.E.SS. L. & E. 
On the 26th of January, about noon, I observed the appearance 
of a coloured bow on the frozen surface of the ditch which sur¬ 
rounds S. John’s College, Cambridge. Its appearance and position 
seemed to correspond with those of an ordinary primary rainbow. 
I at once made a rough measurement of the angle on the board of 
a book which I had with me, and then borrowed from Dr Parkin¬ 
son, President of S. John’s College, a sextant, with which I found 
that the angle between the bright red and the shadow of the large 
mirror was 41° 50', and that for bright blue 40° 307 The angle 
for the extreme red of the primary bow, as given in Parkinson’s 
Optics, is 42° 20 7 , and that for violet 40° 327 The bows formed by 
ice crystals are seen on the same side as the sun, and not on the 
opposite side. I suppose the bow which I saw to be formed by 
small drops of water lying on the ice. If the lower part of 
each drop were flattened, so as to bring the point at which the 
reflexion takes place nearer to the points of incidence and emer¬ 
gence, the effect would be of the same kind as that of a diminution 
of the index of refraction—that is, the angle of the bow would be 
increased. How a drop of water can lie upon ice without wetting 
it, and losing its shape altogether, I do not profess to explain. 
Only a small part of the ice presented this appearance. It was 
best seen when the incident and emergent rays were nearly equally 
inclined to the horizontal. The ice was very thin, and I was not 
able to get near enough to the place where the bow appeared to 
see if the supposed water drops really existed. 
The following Gentlemen were admitted Fellows of the 
Society:— 
W. E. Heathfield, Esq., F.R.G.S., F.C.S. 
Edward James Shearman, M.D., F.R.C.S.L. 
Patrick D. Swan, Esq. 
Dr H. Alleyne Nicholson. 
A ballot also took place for the Rev. Dr Hodson, who resigned 
the Fellowship of the Society in 1867. Dr Hodson was re¬ 
admitted. 
VOL. VII. 
Iv 
