The Constant of Refraction
Publications of the Astronomical Society of the Pacific
The phenomenon known as refraction was first mentioned by and probably discovered by Ptolemy, who lived about the second century A. D. In his famous treatise on optics he points out that the light coming to us from a star or heavenly body, on entering oiir atmosphere and traversing it to its lower and denser portions, is gradually bent or refracted, the result being that the object appears to the observer to be nearer the zenith than it actually is. He shows further that this bending ranges
... bending ranges from zero at the zenith to a maximum at the horizon. Walther, who worked in the fifteenth century A. D., was the first one to make any successful attempt to allow for atmospheric refraction in reducing observations. Tycho, who is celebrated for his wonderful series of accurate observations, recognized fully the importance of refraction, and consequently made a series of observations to find out the amount of displacement of an object, due to this bending for different parts of the sky, and constructed the first table of refraction. Although it was not a very accurate one, it marks an epoch in the study of this phenomenon. Kepler, following Tycho, made a considerable improvement in the theory of astronomical refraction. The law of refraction was discovered by Snell in the early part of the seventeenth century. In the latter part of this same century, Cassini computed a new table of refractions which was an improvement upon Kepler's. This was> in turn, followed by a further improvement by Bradley in the early part of the eighteenth century. . About 1818 Bessel gave us his theory of refraction, which is the one used to-day. "Although the complete theoretical solution was then, as now, unattainable, he succeeded in constructing a table of refractions which agreed very closely with observations, and was presented in such a form that the necessary correction for a star in almost any position could be obtained with very little trouble."