[From the Edinburgh Royal Society Proceedings, Vol. VH.] 



XXXVIII. On a Bow seen on the Surface of Ice. 



UN the 26th of January, about noon, I observed the appearance of a 

 coloured bow on the frozen surface of the ditch which surrounds S. John's 

 C'/ollege, 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 Parkinson, 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 30'. The angle for the extreme red 

 of the primary bow, as given in Parkinson's Optics, is 42 20', and that for 

 violet 40* 32'. 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 emergence, 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 

 tn the place where the bow appeared to see if the supposed water drops really 

 existed. 



