On Dreams   

longer in contact [with the things that are moved]. For that which set

them in motion moves a certain portion of air, and this, in turn,

being moved excites motion in another portion; and so, accordingly, it

is in this way that [the bodies], whether in air or in liquids,

continue moving, until they come to a standstill.

This we must likewise assume to happen in the case of qualitative

change; for that part which [for example] has been heated by something

hot, heats [in turn] the part next to it, and this propagates the

affection continuously onwards until the process has come round to its

oint of origination. This must also happen in the organ wherein the

exercise of sense-perception takes place, since sense-perception, as

realized in actual perceiving, is a mode of qualitative change. This

explains why the affection continues in the sensory organs, both in

their deeper and in their more superficial parts, not merely while

they are actually engaged in perceiving, but even after they have

ceased to do so. That they do this, indeed, is obvious in cases

where we continue for some time engaged in a particular form of

perception, for then, when we shift the scene of our perceptive

activity, the previous affection remains; for instance, when we have

turned our gaze from sunlight into darkness. For the result of this is

that one sees nothing, owing to the excited by the light still

subsisting in our eyes. Also, when we have looked steadily for a

long while at one colour, e.g. at white or green, that to which we

next transfer our gaze appears to be of the same colour. Again if,

after having looked at the sun or some other brilliant object, we

close the eyes, then, if we watch carefully, it appears in a right

line with the direction of vision (whatever this may be), at first

in its own colour; then it changes to crimson, next to purple, until

it becomes black and disappears. And also when persons turn away

from looking at objects in motion, e.g. rivers, and especially those

which flow very rapidly, they find that the visual stimulations

still present themselves, for the things really at rest are then