Holmgren's colored wool test for color blindness. The patient had to match one piece of wool to the samples in the box in this color blindness test. There are light and dark shades to confuse the patient. This helped detect problems. The numbers on the pieces of wool were codes. The doctor used them to determine what type color blindness the patient had. Swedish physiologist Alarik Frithiof Holmgren (1831-1897) devised this test in 1874. He pursued his investigations following a railway accident in Sweden in 1876. The accident was believed to be caused by a color blind train driver. Following Holmgren's research, color blindness tests were made compulsory for railway and shipping workers in Sweden.