Booklet of coloured silk threads, c 1825-1844.

UNITED KINGDOM - JULY 17: This booklet of threads was used by John Dalton (1766-1844) to test his own colour blindness. It was given to him by the astronomer Sir John Frederick William Herschel (1792-1871). Most colour blindness 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. (Photo by SSPL/Getty Images)