Booklet of coloured silk threads, c 1825-1844.
© Science Museum / Science & Society Picture Library
This booklet of threads was used by John Dalton (1766-1844) to test his own colour blindnes. It was given to him by the astronomer Sir John Frederick William Herschel (1792-1871). Most colour blindnes is inherited, and affects more men than women. The condition is also known as Daltonism, and usually involves confusion between red, green and yellow. Dalton formulated the atomic theory to explain chemical reactions, based on the concept that the atoms of different elements are distinguished by differences in their weights. This theory has influenced scientific thought for more than a century and a half.