In Africa's Rift Valley, a simple fly trap is helping to check the spread of the second leading cause of blindness in the world. The inventor David Morley, a retired specialist in tropical child health at Great Ormond Street Hospital in London, has based the device on the assumption that flies fly upwards towards light after feeding. The contraption consists of two bottles, one of which fits into the lower section of the other. The top of the lower bottle is fitted with a tube of narrower girth which guides the flies upwards. The lower bottle is filled with animal dung and urine to attract the flies. Holes facilitate entry of the flies who feed on the mixture and fly upwards through the tube into the next bottle where they die of exhaustion and exposure to ultraviolet light. The number of people suffering from trachoma worldwide is around 14 million. The disease is a bacterial infection spread partly by flies and inflames the eyes. Over the years, a scarring of the cornea brings blindness for the sufferer.