Ozone is a variable trace constituent of the atmosphere of the Earth whose early origin is linked with that of atmospheric oxygen and water vapor. The presence of oxygen initially resulted from photodissociation of water vapor, outgassed from the interior of the Earth and from recombination of water vapor and carbon dioxide in the presence of sunlight. The total amount of ozone in a vertical atmospheric column is determined by a technique first suggested by C. Fabry and M. Buisson and developed for routine measurements by G. M. B. Dobson. This technique provides the basis for most of the ground based observations of total ozone taken in the worldwide total ozone observing network. The Dobson instrument is a double monochrometer which measures the relative radiance at different UV wavelengths after that radiation has suffered extinction by absorption and scattering in the atmosphere. The measurement is taken at a number of wavelength pairs consisting of a short and a long wavelength.