1840. Antoine F. J. Claudet, F.R.S., of London, employed chlorine for the same purpose. -- M. Fizeau, of Paris, deposited a film of gold over the Daguerreotype picture after the removal of the iodine, which imparted increased brilliancy and permanency. -- Chloride of platinum employed by Herschel. -- Fox Talbot's developer published September 20th. 1841. Calotype process patented by Fox Talbot, September 20th. -- First photographic compound portrait lens made by Andrew Ross, London. -- Towson, of Liverpool, noted that chemical and visual foci did not coincide. Defect corrected by J. Petzval, of Vienna, for Voightlander. -- "A Popular Treatise on the Art of Photography, including Daguerreotype and all the New Methods of Producing Pictures by the Chemical Agency of Light," by Robert Hunt, published by R. Griffin, Glasgow. -- Daguerre announced an instantaneous process, but it was not successful. 1842. Sir John Herschel exhibited blue, red, and purple photographs at the Royal Institution. -- "Photography Familiarly Explained," by W. R. Baxter, London. 1843. "Photogenic Manipulation," by G. T. Fisher Knight, Foster Lane. -- Treatise on Photography by N. P. Lerebours, translated by J. Egerton. 1844. Fox Talbot issued "The Pencil of Nature," a book of silver prints from calotype negatives. -- C. Cundell, of London, employed and published the use of bromide of potassium in the calotype process. 1844. "Researches on Light and its Chemical Relations," by Robert Hunt. First edition; second ditto, 1854. -- Robert Hunt recommended proto-sulphate of iron as a developer for Talbot's calotype negatives; also oxalate of iron and acetate of lead for other purposes. -- A. F. J. Claudet patented a red light for "dark room," but at that date a red light was not necessary, so the old photographers continued the use of yellow lights. 1845. "Photogenic Manipulations:" Part 1, Calotype, &c.; Part 2, Daguerreotype. By George Thomas Fisher, jun. Published by George Knight and Sons, London. -- "Manual of Photography," including Daguerreotype, Calotype, &c., by Jabez Hogg. First edition. Second ditto, including Archer's collodion process, bichloride of mercury bleaching and intensifying, and gutta-percha transfer process, 1856. 1845. "Practical Hints on the Daguerreotype; Willats's Scientific Manuals." -- "Plain Directions for Obtaining Photographic Pictures by the Calotype and other processes, on paper; Willats's Scientific Manuals." Published by Willats, 98, Cheapside; and Sherwood, Gilbert, and Piper, Paternoster Row. 1846. Gun-cotton made known by Professor Schönbein, of Basel. 1847. Collodion made by dissolving gun-cotton in ether and alcohol, by Mr. Maynard, of Boston, U.S. 1848. "Photogenic Manipulation:" Part II., Daguerreotype, by Robert Bingham. Published by George Knight and Sons, London. -- Albumen on glass plates first employed for making negatives by M. Niépce de Saint Victor. Process published June 13th.