These are obtained as follows: A sheet of paper is coated with a ten per cent solution of gelatine, and when dry this is floated on a ten per cent solution of bichromate of potash. Again dry and expose beneath a positive transparency. The print thus obtained is then immersed in a ten per cent solution of chloride of cobalt. The parts unacted upon by light will absorb the solution. Wash and dry. We then have a faint image which will alter its color according to the state of the atmosphere. In damp weather it will be almost if not entirely invisible, but when the weather is fine and dry, or if the image be heated before a fire it will turn to a bright blue color.