The advance of photography : its history and modern applications (1911)

THE CHEMICAL ACTION OF LIGHT 67 exposed to light under a negative, then plunged in sulphocyanide of potassium, and ultimately treated with red prussiate of potash. The result produced by this somewhat complicated process was a brown picture. Action of Light on Glass. — The celebrated physicist Faraday made the observation that flesh-coloured glass, which is stained with manganese, became rapidly brown in the light. This fact remained for a long time isolated, until, years later, other observations of the same kind were made. A very handsome plate of glass was exhibited in a mirror shop at Berlin. It bore the inscription " Spiegelmanufactur " in brass letters. After being exhibited for years, the business was broken up, and the mirror, on account of its beauty, was taken away by its owner, the brass letters were removed, and the plate cleaned. To the surprise of the proprietor, the letters remained plainly visible on the glass, notwithstanding all attempts to remove them. The surface was even ground away, but this did not produce any effect on the letters. It was found that the glass had become yellow throughout, and that it remained white only at the places where the opaque letters had kept off the light. The plate of glass was cut into halves. One half, with the word " Spiegel," was placed in the Physical Museum of the University of Berlin. Some very interesting observations on the action of light on glass were made by Gaffield, who found that almost all sorts of glass are affected by light, and that in many cases an exposure of only a few days suffices to produce a change. Gaffield cut the glass to be examined into two parts, placed one in the dark and the other in the light, and compared the two after a few days. In almost all cases he remarked that the colour of the glass became deeper on exposure to light. Two kinds only of German and Belgian green window glass were unaffected. The exposed glasses