Two essays : one upon single vision with two eyes ; the other on dew. A letter to the Right Hon. Lloyd, Lord Kenyon and an account of a female of the white race of mankind, part of whose skin resembles that of a negro ; with some observations on the causes of the differences in colour and form between the white and negro races of men. By the late William Charles Wells, M.D. F.R.S. L. & E. with a memoir of his life, written by himself