In early studies of the sun’s spectrum it was found that the atmosphere diminishes the intensity of solar radiation. In general the effect is larger in the blue or violet than at longer wavelengths. This is why a scene viewed through a red filter, such as red cellophane, appears to be so “clear”. It was further discovered in the nineteenth century that the diminishing effect of the atmosphere can actually be more complicated than a smooth blue-to-red gradient and that certain wavelengths of sunlight are strongly absorbed by individual trace gases in specific wavelength bands and, moreover, to complicate things even further the diminution or “turbidity” was found to vary from one day to another. Assessing this “diminution” nuisance became especially important in quantifying incoming solar radiation reaching the earth. The investigation of solar radiation reaching the earth became a subject of great interest because it was suspected that the sun may undergoes changes in its luminosity that would modulate climate and weather and the growing of crops.