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On Sense And The Sensible 
part, is manifestly smooth. The phenomenon of the flash occurs only
when the eye is moved, because only then could it possibly occur
that the same one object should become as it were two. The rapidity of
the movement has the effect of making that which sees and that which
is seen seem different from one another. Hence the phenomenon does not
occur unless the motion is rapid and takes place in darkness. For it
is in the dark that that which is smooth, e.g. the heads of certain
fishes, and the sepia of the cuttle-fish, naturally shines, and,
when the movement of the eye is slow, it is impossible that that which
sees and that which is seen should appear to be simultaneously two and
one. But, in fact, the eye sees itself in the above phenomenon
merely as it does so in ordinary optical reflexion.
If the visual organ proper really were fire, which is the doctrine
of Empedocles, a doctrine taught also in the Timaeus, and if vision
were the result of light issuing from the eye as from a lantern, why
should the eye not have had the power of seeing even in the dark? It
is totally idle to say, as the Timaeus does, that the visual ray
coming forth in the darkness is quenched. What is the meaning of
this 'quenching' of light? That which, like a fire of coals or an
ordinary flame, is hot and dry is, indeed, quenched by the moist or
cold; but heat and dryness are evidently not attributes of light. Or
if they are attributes of it, but belong to it in a degree so slight
as to be imperceptible to us, we should have expected that in the
daytime the light of the sun should be quenched when rain falls, and
that darkness should prevail in frosty weather. Flame, for example,
and ignited bodies are subject to such extinction, but experience
shows that nothing of this sort happens to the sunlight.
Empedocles at times seems to hold that vision is to be explained
as above stated by light issuing forth from the eye, e.g. in the
following passage:-
As when one who purposes going abroad prepares a lantern,
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