Because of the actinic powers that accompany the solar light.
556. What is the particular chemical effect of light exhibited in the production of photographic pictures?
Simply the darkening of preparations of silver, by the actinic rays.
557. Why are photographic studios usually glazed with blue glass?
Because blue glass obstructs many of the luminous rays, but it is perfectly transparent to actinism.
"The hay appeareth, and the tender grass showeth itself, and herbs of the mountain are gathered."—Prov. xxvii.