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.