About ten men in every hundred suffer from colour blindness in some way; women are luckier only about one in two hundred is affected in this manner. There are different forms of colour 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 colour 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! Colour 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 colours. There are also millions of “rods” but these are used for seeing when it is nearly dark. They show us shape but not colour. Wait until it is dark tonight, 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 colours. As far as we know, bats and adult owls cannot see colours at all only light and dark shapes. Similarly cats and dogs cannot see colours 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 grey and dull to us, but to insects they may appear beautiful, showing colours which we cannot see. Scientists know that there are other colours around us which insects can see but which we cannot see. Some insects have favorite colours. Mosquitoes like blue, but do not like yellow. A red light will not attract insects but a blue lamp will.