Sir John Herschel Invents Cyanotypes, the Basis for Blueprints

In 1842 English mathematician, astronomer, chemist, and experimental photographer/inventor Sir John Herschel, invented the cyanotype, a photographic process that resulted in a cyan-blue print.

"The photosensitive compound, a solution of ferric ammonium citrate and potassium ferricyanide, is coated onto paper. Areas of the compound exposed to strong light are converted to insoluble blue ferric ferrocyanide, or Prussian blue. The soluble chemicals are washed off with water leaving a light-stable print."

The process was used through the 20th century by architects and engineers for the production of blueprints.