! G. Cowan, Exercises in Statistical Data Analysis ! ! Data of Claudius Ptolemy on refraction, from Olaf Pedersen and ! Mogens Pihl, "Early Physics and Astronomy: a Historical Introduction", ! MacDonald and Janes, London, 1974. ! ! angle of angle of ! incidence refraction (degrees) ! 10 8 20 15.5 30 22.5 40 29 50 35 60 40.5 70 45.5 80 50