On 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 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 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.