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.
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.
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.
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.
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.