Do We See Something as Soon as We Look at it?
Certainly not. Seeing, like every other kind of sensation, takes time. From the instant that the light strikes the curtain at the back of our eyes to the instant that we see is quite a long time compared with some things - for instance, compared with the time it takes light to travel a mile.
We are likely to think a second the shortest part of time that is worth mentioning, but that is absurd. A second is a period of time so long that light, radiant heat and electricity could travel almost as far as the moon in such a time.
The wonderful thing about seeing is that it takes such a small fraction of a second for all the things to happen which are necessary before we can see. Complicated chemical changes have to take place in the living cells of the curtain at the back of the eye. These changes have to produce special nerve- currents, which in some amazing way correspond exactly to them; and these run along the eye-nerves, first to a group of cells in the lower part of the brain, and then from them, along another set of nerves, to the real eyes - a group of nerve-cells at the very back of the brain, which have developed, and live, in utter darkness. Something happens in them, and then we say we see. The marvel is that only a few hundredths of a second are needed for so much to happen.
