Radiation can only affect matter if absorbed by it. Within the broad range of 300-1000 nm, which we call "the visible," light quanta are energetic enough to produce excited electronic states in the atoms and molecules that absorb them. In these states the molecules may have quite different properties from those in their dormant condition, and reactions that would not otherwise occur become possible. About 80 % of the radiant energy emitted by our sun lies in this fertile band, and so long as the sun's surface temperature is maintained at about 6000 C this state of affairs will continue. This...