Determinations of sky-blue

The determination of the color of the sky entered into modern meteorology with the introduction of the Blue-Scale by Linke and Ostwald [see 1 of references at end of paper], although de Saussure [2] had already made observations of that kind. The intimate relation between the blue color of the sky and the turbidity within the atmosphere, hence with such important elements as the intensity of radiation, the suspended impurities, and the visual range, made surveys of the sky-color desirable. The blue-scale, consisting of a set of cardboards with different shades of blue, is a very handy and cheap device, but its usefulness suffers from certain difficulties in operation. The matching of scale and sky-blue, in particular, often makes trouble, because of the discontinuities in the steps of the cardboard-scale. A simple instrument was therefore constructed, replacing the cardboard-scale by a glass-wedge. This wedge, made of the Signal Blue No. 556 of the Corning Glass Works, had its maximum transmission at about 425 millimicrons. It was movable by a sledge across an eyepiece, so that half of the field was open to the sky. The wedge was illuminated by a mirror, which reflected the sunlight. The thickness of the wedge varied from 0.025 to 0.584 cm on a length of 15.3 cm. In our observations a thickness of 0.23 cm was rarely exceeded to match the sky-blue. By mounting the instrument on a support with graduated circles the azimuth and altitude for each measurement could be determined.