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 matter. So some people say it is safer to be driven by women. There are different forms of colour blindness. In some cases 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. Colour blindness in humans is a strange thing to explain. In our eyes there are millions of very small things called "cones". These help us to see in a bright light and to tell differences between colours. There are also millions of "rods", but these are used for seeing when it is nearly dark. They show us shapes but no colour. Some insects have favourite colours. Mosquitoes like blue but do not like yellow. A red light will not attract insects, but a blue lamp will. In a similar way humans also have favourite colours. Yet we are lucky. With the aid of the cones in our eyes we can see many beautiful colours by day, and with the aid of the rods we can see shapes at night. One day we may even learn more about the invisible colours around us. This passage is mainly about  _
colour and its surprising effects danger caused by colour blindness colour blindness the invisible colours around us