This book is about how light was made to count. It explores a seemingly simple question: How was the brightness of light—casually judged by everyone but seldom considered a part of science before the 20th century—transformed into a measurable and trustworthy quantity? Why did the description of colour become meaningful to artists, dyers, industrialists and a handful of scientists? Seeking answers requires the exploration of territory in the history, sociology and philosophy of science. Lightwasmade to count as a quantifiable entity at the same time as it came to count for something in human terms. Measuring the intensity of light was fraught with difficulties closely bound up with human physiology, contentious technologies and scientific sub-cultures.