On a quiet afternoon in 1794, John Dalton stood in his laboratory in Manchester, staring at a piece of blue fabric. To his eyes, it looked exactly like the green cloth beside it. He had asked his assistant to bring him a sample of each colour, but the young man insisted they were different. Dalton shook his head, puzzled. He had always assumed everyone saw colours the same way. But now, at the age of twenty-eight, he began to suspect that his own vision was unusual. That moment of confusion sparked a curiosity that would lead him to describe a condition later known as colour blindness—and to change how scientists understood human perception.

Dalton was born in 1766 in a small village in England's Lake District. His family were Quakers, and he grew up in a modest home where education was valued. He showed an early talent for mathematics and science, and by the age of twelve he was already teaching at a local school. His father, a weaver, could not afford to send him to university, so Dalton continued teaching and studying on his own. He moved to Manchester in 1793, where he became a tutor at New College. There, he began to conduct experiments in meteorology and chemistry, keeping detailed records of the weather every day for over fifty years.

The turning point came when Dalton tried to explain his colour confusion. He asked friends and colleagues to describe the colours of objects, and he recorded their answers. He noticed that his brother also saw colours differently, which suggested the condition was inherited. Dalton hypothesised that the fluid inside his eyeball was tinted blue, filtering out certain wavelengths of light. He even requested that after his death, his eyes be dissected to test this idea. The dissection showed no blue tint, but his careful documentation laid the groundwork for future research into colour vision deficiency.

There, he began to conduct experiments in meteorology and chemistry, keeping detailed records of the weather every day for over fifty years.

Dalton's response to his discovery was methodical and determined. He wrote a paper for the Manchester Literary and Philosophical Society, describing his own visual experiences in precise detail. He explained that he could not distinguish between red and green, and that some colours he saw as shades of yellow or blue. The paper was published in 1798 and attracted widespread attention. Dalton did not stop there; he continued to study the phenomenon, corresponding with other scientists and refining his theories. His work helped establish the idea that perception could vary from person to person, a radical notion at the time.

Despite his growing fame, Dalton remained a humble and dedicated teacher. He never married and lived a simple life, spending most of his time in his laboratory or classroom. He was known for his eccentric habits, such as wearing a broad-brimmed hat and carrying a walking stick. He also had a deep love for meteorology, and his daily weather records eventually filled over 200,000 observations. In 1803, he published his atomic theory, which revolutionised chemistry. But he never forgot his early curiosity about colour. He often joked that he could not tell a ripe tomato from a green one, and he relied on his friends to help him choose matching clothes.

Dalton's impact on science is immense. His description of colour blindness—often called "Daltonism" in his honour—opened the door to understanding sensory differences. Today, about one in twelve men have some form of colour vision deficiency, and Dalton's careful self-study remains a classic example of scientific observation. A fun fact: Dalton was so meticulous that he kept a pair of coloured stockings that he thought were grey, but which his housekeeper insisted were bright red. He wore them to a Quaker meeting, causing quite a stir. That small, embarrassing moment reminds us that even great scientists can be surprised by their own limitations—and that asking questions about the ordinary can lead to extraordinary discoveries.