填空题

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. When millions of rods in our eyes are at work in darkness, we can see ______.