‎"HERSCHEL, JOHN W.F. - DISCOVERY OF PRUSSIAN BLUE IN PHOTOGRAPHIC PROCESSES.‎
‎On the Action of the Rays of the Solar Spectrum on Vegetable Colours, and on some new Photographic Processes. Received June 15, - Read June 16, 1842.‎

‎(London, Richard and John E., Taylor, 1842.). 4to. No wrappers as extracted from ""Philosophical Transactions"" 1842 - Part II. Pp. 181-214 and one double-page folded engraved plate.‎

Reference : 42843

‎First appearance of a pioneer-paper in the history of early photography in which Herschel announced some of his importent discoveries of the photographic printing processes, the process of photographic contact-printing in Prussian blue, brought to light just three years after Louis Daguerre and Henry Talbot had announced their independent inventions of photography in silver, using metal and paper substrates, respectively. and the photographic properties of red ferro sesquicyanuret of potassium.""This is the first recorded observation of Prussian blue being formed for a photographic purpose by the action of light on potassium ferricyanide, so it represents the moment of discovery of the first cyanotype process, although this name still lay in the future. The significance of this observation impressed Herschel sufficiently to mention it also in his general diary entry for 23 April 1842....""(Mike Ware in ""John Herschel's Cyanotype. Invention or discovery ?"").‎