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68 THE ADVANCE OF PHOTOGRAPHY
recovered their original colour on being heated to redness.
This alteration of glass by light has a most injurious effect in photography. The yellow tinge, which the glass gradually assumes, absorbs a part of the chemically active light. This absorption makes itself strikingly evident, because the time of exposure for photographs must be continually lengthened.
The greatest change is produced in glass containing manganese. Peroxide of manganese, or black manganese, is frequently added to glass to bleach it. The oxygen of the peroxide converts the dark-green ferrous oxide contained in the glass into the lighter ferric oxide. In the light the converse reaction takes place. The ferric oxide is reduced to ferrous oxide, the oxygen combines again with the manganese to form black manganese, and thus the colour of the glass becomes deeper.
In many minerals the opposite effect to that in glass is produced on exposure to light ; their colour becomes paler instead of deeper. This is the case with the Siberian topaz, which soon loses its golden-yellow colour in the light. A splendid crystal of topaz, six inches long, belonging to the Mineralogical Museum at Berlin, has in this manner lost much of the beauty of its appearance.
Action of Light having low Actinic Value. — Some very curious effects have been observed when light of which the actinic value is small has been allowed to act upon photographic plates and papers. The following, due to the researches of P. Villard,1 are particularly interesting.
If a print is made on albuminised or salted paper by exposing for a few seconds in daylight until the darkest parts of the print just begin to appear, then the paper is taken out and exposed in sunlight under green or yellow glass, which completely cuts off the blue and ultra-violet
1 Soc. Franc. Phys. Bull, No. 1, pp. 5-27, 1907, and Journ. de Physique, 6, pp. 360-379, May, and pp. 445-457, June 1907.