Because rays of light flow from their bodies to each other's eyes, and convey an impression of their respective conditions.
In some popular works that have come under our notice, we find that the student is told that "we cannot absolutely see each other—we only see the rays of light reflected from each other." The statement is erroneous as expressed. We do not see the rays of light, for if we did so, the effect of vision would be destroyed, and all bodies would appear to be in a state of incandesence, or of phosphoresence. Rays of light, which are in themselves invisible, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object—what is? It would be just as reasonable to say, that we cannot hear a person speak—that we only hear the vibrations of the air. But as the vibrations are imparted to the air by the organs of voice of the speaker, as he sets the air in motion, and makes the air his messenger to us, we certainly hear him, and can dispense with any logical myths that confound the understanding, and contribute to no good result.
Actinism is the chemical property of light.
Actinism—ray power.
553. Why does silver tarnish when exposed to light?
Because of the actinic, or chemical power of the rays of the sun.
554. Why do some colours fade, and others darken, when exposed to the sun?
Because of the chemical power of the sun's rays.