[2] With the Greeks the word “Optics” or Ὀπτικά (from ὄπτομαι, the obsolete present of ὁρῶ, I see) was restricted to questions concerning vision, &c., and the nature of light.
[3] It seems probable that spectacles were in use towards the end of the 13th century. The Italian dictionary of the Accademici della Crusca (1612) mentions a sermon of Jordan de Rivalto, published in 1305, which refers to the invention as “not twenty years since”; and Muschenbroek states that the tomb of Salvinus Armatus, a Florentine nobleman who died in 1317, bears an inscription assigning the invention to him. (See the articles [Telescope] and [Camera Obscura] for the history of these instruments.)
[4] Newton’s observation that a second refraction did not change the colours had been anticipated in 1648 by Marci de Kronland (1595-1667), professor of medicine at the university of Prague, in his Thaumantias, who studied the spectrum under the name of Iris trigonia. There is no evidence that Newton knew of this, although he mentions de Dominic’s experiment with the glass globe containing water.
[5] The geometrical determination of the form of the surface which will reflect, or of the surface dividing two media which will refract, rays from one point to another, is very easily effected by using the “characteristic function” of Hamilton, which for the problems under consideration may be stated in the form that “the optical paths of all rays must be the same.” In the case of reflection, if A and B be the diverging and converging points, and P a point on the reflecting surface, then the locus of P is such that AP + PB is constant. Therefore the surface is an ellipsoid of revolution having A and B as foci. If the rays be parallel, i.e. if A be at infinity, the surface is a paraboloid of revolution having B as focus and the axis parallel to the direction of the rays. In refraction if A be in the medium of index µ, and B in the medium of index µ′, the characteristic function shows that µAP + µ′PB, where P is a point on the surface, must be constant. Plane sections through A and B of such surfaces were originally investigated by Descartes, and are named Cartesian ovals. If the rays be parallel, i.e. A be at infinity, the surface becomes an ellipsoid of revolution having B for one focus, µ′/µ for eccentricity, and the axis parallel to the direction of the rays.
[6] Young’s views of the nature of light, which he formulated as Propositions and Hypotheses, are given in extenso in the article [Interference]. See also his article “Chromatics” in the supplementary volumes to the 3rd edition of the Encyclopaedia Britannica.
[7] A crucial test of the emission and undulatory theories, which was realized by Descartes, Newton, Fermat and others, consisted in determining the velocity of light in two differently refracting media. This experiment was conducted in 1850 by Foucault, who showed that the velocity was less in water than in air, thereby confirming the undulatory and invalidating the emission theory.
[8] Newton, Opticks (London, 1704).
[9] Trans. Irish Acad. 15, p. 69 (1824); 16, part i. “Science,” p. 4 (1830), part ii., ibid. p. 93 (1830); 17, part i., p. 1 (1832).
[10] This kind of type will always be used in this article to denote vectors.
[11] Phil. Trans. (1802), part i. p. 12.