WILLIAM S. HILLMAN ¹

Rather than concentrate on a single small aspect of light physiol-
ogy, I shall try to review two general topics and to point out
areas that may have been neglected.

Let us immediately exclude from consideration both photosynthe-
sis and vision. These have an immense literature and are relatively
--though only relatively--well understood. At least all biologists,
whatever their special interests, are aware that photosynthesis and
vision exist, which is more than can be said for many other light
effects. For similar reasons let us also exclude ultraviolet effects--
wavelengths below 4000 Å--although they will enter slightly a bit
later.

It has become clear to botanists only relatively recently that
many actions of light, even on green plants, bear no close relation-
ship to photosynthesis; zoologists occasionally still seem to assume
that all effects of light on animals are necessarily mediated through
vision, and some microbiologists seem unaware that light can affect
many of their experimental organisms. This brief paper will have
served its purpose if it merely emphasizes the multiplicity and
widespread nature of light effects, as well as our ignorance concern-
ing most of them.

At least two major questions are asked by the light physiologist.
The first is: What is the compound absorbing the light involved in
a particular process--the photoreceptor pigment or pigments? The
second is: What processes intervene between light absorption and

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