There are two kinds of sensory cells in our eyes–the rods and the cones–and they have very different
functions. The cones, which I call ladybird beetle cells, are very sharp about color and detail, but very stupid about dim
objects. On the other hand the rods are totally stupid about color and detail but are fairly smart about very dim objects.
And, as you must have noticed, in the daytime, with your cone cells, you can see the leaves on the trees. But at night, with
your rod cells, you see only the trees. That’s because your cone cells go off duty in the dark.
Now, to do their job nicely your rod cells need some help. Their effectiveness is spoiled by exposure to
bright light. That’s why you can’t see very well at night just after someone shines a flashlight in your eyes.
The bright light does two things to your eyes. It closes down the aperture of your pupils which let the light in, and it bleaches
out a chemical which the rods use for seeing in the dark. And although the aperture of the pupil can be quickly readjusted,
the replacement of the chemical takes some time.
The eyes produce a light-sensitive pigment called rhodopsin. It is also called visual purple. The rod cells use it to help us see in the dark, and it’s
made out of vitamin A. That’s why we’re told to eat carrots to improve our night vision. However, you can get
only about one percent of the vitamin A out of carrots by munching them down raw. You can get nearly twenty percent by cooking
them, and over ninety percent by juicing them. But spinach and other thin-green leafy vegetables have lots more vitamin A
than do carrots.
The eyes continually produce rhodopsin, but in the daytime it is continually bleached out by light. And
it takes quite a long time for the rhodopsin to build up to full capacity. That is why, when someone shines a bright light
in your eyes at night, it takes quite a while before you can see as well as you could see before. You may have noticed that
sometimes, when you wake up very late at night, the room looks a lot brighter than it did when you went to bed. And although
there may have been no change in the actual brightness of the room, it appears brighter because the rhodopsin has accumulated
in your eyes and increased your ability to see in dim light. We say that your eyes have become dark adapted. And it has been pointed out that total adaptation to the dark may take some forty-eight hours.
It has often been suggested that when you are looking through a telescope at very dim objects, you should go to very
low power to make the image as bright as possible. But this advice is extremely misleading. It completely overlooks the fact
that very dim objects can be seen only with the rod cells. And it overlooks the fact that the rod cells are stupid about detail.
If the object is too dim to be seen by the cone cells, it is hopeless to go to low power. Try it yourself.