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  1. Presidential Address before the Optical Society of America, in Rochester, N. Y., October 24, 1921; on the occasion of the Helmholtz Memorial Celebration.
  2. See a recent article on "The Law of Refraction" by Dr. R. A. Houstoun in Science Progress, xvi (1922), pp. 397–407; where is given an interesting account of the experimental methods of Claudius Ptolemy for measuring the angles of refraction corresponding to given angles of incidence, with a table showing the actual values which he obtained for air-water, air-glass and water-glass. Further on in the same paper, Dr. Houstoun tells about Kepler's work in this field and gives Kepler's refraction-formula in the form: [Equation] where i and r denote the angles of incidence and refraction, and µ the relative index of refraction; which (he adds) "like the modern formula, sin i = µ sin r, is a one-constant formula." Houstoun subjoins a table of numerical values showing how closely this formula of Kepler's corresponded with the results of his measurements and with the earlier measurements made by Vitellio.
  3. Neither Galileo nor Scheiner can be said to be the original discoverer of the spots on the Sun, because as a matter of fact these appearances had been noted by the Chinese with the naked eye as long ago as the beginning of the fourth century (301 A.D.). Galileo seems to have seen them for the first time in August 1610, whereas Scheiner did not see them until March 1611. Meanwhile, however, they had been also observed entirely independently by a certain Johann Fabricius in December 1610, who published a book in June 1611 entitled "Narratio de Maculis in Sole Obser-vatus," wherein the phenomena were minutely described. This was the first announcement in print; but neither Galileo nor Scheiner appears to have had the slightest knowledge of Fabricius's work. Scheiner's first letter on the subject, dated November 12, 1611, was published January 5, 1612. This was followed by Galileo's letter dated May 4, 1612 but not published until the following year.
  4. Eleven years after the death of Snell, Descartes published his Dipotrics, in which he announced the law of refraction in terms of sines as we have it now, without mentioning Snell's name. It is said that Descartes had access to Snell's manuscript; Huygens states that he himself had seen the whole manuscript volume of Snell, and that he believed that Descartes had also seen it. "Biot attempts to give the whole glory of the discovery to his countryman Descartes, but the preponderance of opinion is in favour of the view that Descartes' discovery was not independent." While it is probable that this is a question which can never be settled with certainty; it may be added that "Descartes was not generous in speaking of the work of others. In dealing with the invention of the telescope, for example, he does not mention the name of Galileo, and in treating of the rainbow he makes no reference to Antonio de Dominis." See R. A. Houstoun, "The Law of Refraction," Set. Prog., xvi (1922), pp. 397–407.

Houstoun, R. A.

See a recent article on "The Law of Refraction" by Dr. R. A. Houstoun in Science Progress, xvi (1922), pp. 397–407; where is given an interesting account of the experimental methods of Claudius Ptolemy for measuring the angles of refraction corresponding to given angles of incidence, with a table showing the actual values which he obtained for air-water, air-glass and water-glass. Further on in the same paper, Dr. Houstoun tells about Kepler's work in this field and gives Kepler's refraction-formula in the form: [Equation] where i and r denote the angles of incidence and refraction, and µ the relative index of refraction; which (he adds) "like the modern formula, sin i = µ sin r, is a one-constant formula." Houstoun subjoins a table of numerical values showing how closely this formula of Kepler's corresponded with the results of his measurements and with the earlier measurements made by Vitellio.

Other (4)

Presidential Address before the Optical Society of America, in Rochester, N. Y., October 24, 1921; on the occasion of the Helmholtz Memorial Celebration.

See a recent article on "The Law of Refraction" by Dr. R. A. Houstoun in Science Progress, xvi (1922), pp. 397–407; where is given an interesting account of the experimental methods of Claudius Ptolemy for measuring the angles of refraction corresponding to given angles of incidence, with a table showing the actual values which he obtained for air-water, air-glass and water-glass. Further on in the same paper, Dr. Houstoun tells about Kepler's work in this field and gives Kepler's refraction-formula in the form: [Equation] where i and r denote the angles of incidence and refraction, and µ the relative index of refraction; which (he adds) "like the modern formula, sin i = µ sin r, is a one-constant formula." Houstoun subjoins a table of numerical values showing how closely this formula of Kepler's corresponded with the results of his measurements and with the earlier measurements made by Vitellio.

Neither Galileo nor Scheiner can be said to be the original discoverer of the spots on the Sun, because as a matter of fact these appearances had been noted by the Chinese with the naked eye as long ago as the beginning of the fourth century (301 A.D.). Galileo seems to have seen them for the first time in August 1610, whereas Scheiner did not see them until March 1611. Meanwhile, however, they had been also observed entirely independently by a certain Johann Fabricius in December 1610, who published a book in June 1611 entitled "Narratio de Maculis in Sole Obser-vatus," wherein the phenomena were minutely described. This was the first announcement in print; but neither Galileo nor Scheiner appears to have had the slightest knowledge of Fabricius's work. Scheiner's first letter on the subject, dated November 12, 1611, was published January 5, 1612. This was followed by Galileo's letter dated May 4, 1612 but not published until the following year.

Eleven years after the death of Snell, Descartes published his Dipotrics, in which he announced the law of refraction in terms of sines as we have it now, without mentioning Snell's name. It is said that Descartes had access to Snell's manuscript; Huygens states that he himself had seen the whole manuscript volume of Snell, and that he believed that Descartes had also seen it. "Biot attempts to give the whole glory of the discovery to his countryman Descartes, but the preponderance of opinion is in favour of the view that Descartes' discovery was not independent." While it is probable that this is a question which can never be settled with certainty; it may be added that "Descartes was not generous in speaking of the work of others. In dealing with the invention of the telescope, for example, he does not mention the name of Galileo, and in treating of the rainbow he makes no reference to Antonio de Dominis." See R. A. Houstoun, "The Law of Refraction," Set. Prog., xvi (1922), pp. 397–407.

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