John Dalton, an eighteenth-century chemist, spent time working on the cause of color blindness, which he and his brother both experienced. To both men, red and green appeared to be the same color. Dalton incorrectly hypothesized that the problem was in the color of the fluid that filled his eye. We now know that he was missing cones that are important for telling the difference between red and green light. How do we know that Dalton had a normal amount of S opsin cones?