About ten men in every hundred suffer from color blindness in
some way; women are luckier only about one in two hundred is affected in this
manner. There are different forms of color blindness. A man may not be able to
see deep red. He may think that red, orange and yellow are all shades of green.
Sometimes a person cannot tell the difference between blue and green. In rare
cases an unlucky man may see everything in shades of green--a strange world
indeed. In certain occupations color blindness can be dangerous
and candidates are tested most carefully. For example, when fighting at night,
soldiers use lights of flares to signal to each other. A green light may mean
"Advance" and a red light may mean "Danger! Keep back!" You can see what will
happen if somebody thinks that red is green! Color blindness in human beings is
a strange thing to explain. In a single eye there are millions of very small
things called "cones". These help to see in a bright light and to tell the
difference between colors. There are also millions of "rods" but these are used
for seeing when it is nearly dark. They show us shape but not color. Wait until
it is dark tonight, and then go outside. Look round you and try to see what
colors you can recognize. Birds and animals which hunt at night
have eyes which contain few or no cones at all, so they cannot see colors. As
far as we know, bats and adult owls cannot sec colors at all only light and dark
shapes. Similarly cats and dogs cannot see colors as well as we can.
Insects can see ultra-violet rays which are invisible to us, and some of
them can even see X-rays. The wings of a moth may seem gray and dull to us, but
to insects they may appear beautiful, showing colors which we cannot see.
Scientists know that there are other colors around us which insects can see but
which we cannot see. Some insects have favorite colors. Mosquitoes like blue,
but do not like yellow. A red light will not attract insects but a blue lamp
will. Why can’t bats and adult owls see colors according to the passage