. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. SNELLIUS—LAW OF EEFRACTION. 109 stancy, at a comparatively recent period, marks an era in the history of the science ; and it Avas, as we shall see, the discovery of the laiv of refraction. TIk; ascertained index of refraction for water is If we m:dce a com- pntation of its value from the measured angles of Ptolemy, we find a mean of But if we take his measurements at the incidence of SO-", where the relative variatio
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. SNELLIUS—LAW OF EEFRACTION. 109 stancy, at a comparatively recent period, marks an era in the history of the science ; and it Avas, as we shall see, the discovery of the laiv of refraction. TIk; ascertained index of refraction for water is If we m:dce a com- pntation of its value from the measured angles of Ptolemy, we find a mean of But if we take his measurements at the incidence of SO-", where the relative variations of the angles of incidence and refraction are most marked and most easily measured, we obtain , which is exceedingly near the truth. The true index of refraction for glass is between and , according to the materials and density. Crown glass varies from below to about Ptolemy's mean determination would be But at 50° he approaches nearer the truth, his angles giving For rays passing from water to glass, the relative index computed from his measurements would be , the true being The near agreement of these numbers with niodcrn determinations is remarkable, cspecnally consid- ering that Ptolemy's measures arc given only to the nearest half degree. Ptolemy was unable, however, to derive any practical advantage from these results, since the magnitudes of the angles seemed to be governed by no law Avhich he could detect. And in this unsatisfactory condition the whole subject of refraction remained for the fifteen succeeding centuries. As an astronomer, Ptolemy could hardly fail to notice the effect of atmos- pheric refraction upon the ai)parent positions of the heavenly bodies; and he has the merit of having recognized the fact, which others after him disputed, that the displacement is always in a vertical plane, and also that it attains its maximum in the horizon and is zero in the zenith. About half a century later than
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