There are two types of sensory cells present on our retina: rods and cones. At twilight more than 100 million rods permit us to distinguish between the colourless bright light and the dark. During the day around 6 million cones help us see a variety of colours. The eyes of dogs possess very few cones, which are sensitive only to blue and yellow. For this reason, dogs see the world differently from us. Even there are many people who are blind to red and green and cannot correctly distinguish between these two colours.
