AN optical phenomenon is reported by a correspondent from Elobey Island, lat. 1° N., long. 9° 30′ E., in the Gulf of Guinea. On October 11, 1911, between 1 and 2 p.m., he observed” a large light, of different colours as the rainbow, encircling the sun, and at times only visible on the east side and sometimes only on the west of the sun, and at 2 p.m., our time, disappeared altogether. During this time the sky was covered with swiftly passing small clouds, and shortly after the disappearance of the phenomenon heavy rains began to fall. Without information as to the angular diameter of the ring or the order of the colours it is not possible to say with certainty whether it was a halo or a corona, but its appearance with low clouds makes it probable that the phenomenon was a corona. The corona sometimes appears round the sun when it shines through thin cloud or mist. It is coloured, red being outermost, and several successive sets of coloured rings are usually formed. They are due to the diffraction which the light undergoes in passing among drops of which the cloud is composed. The radius of the first ring of the corona varies from 1° to 3°, according to the size of the drops, and radii of the others are successive multiples of that of the first. As the drops of water in the mist or cloud become larger the rings grow smaller. Their diminution consequently implies approaching rain.