ORIGIN AND HISTORY 



others. He established the fact that refrac- famous Discours de la Methode in Holland in 



tion does not depend on the intensity of light. 1637. It is in its main part a philosophical 



Quantitatively, he thought that for the ratio treatise which established the fame of its 



of the angles of incidence and refraction a author all over Europe but it contains also 



relation involving the secant of the angle of essays on geometry, meteors, and optics in 



incidence would hold. which applications the usefulness of his 



The anatomical structure of the eye was method is demonstrated. In the optical part, 



unknown at Kepler's time. He made the dis- called Dioptrice or Diopfrique, Descartes 



CO very that the image of an object observed strives at a deeper understanding of optical 



is focused on the retina by the refractive experiments from mechanical considerations, 



power of the crystalline lens. He is also Already Hero of Alexandria had felt the 



among the first to arrive at distinct prescrip- necessity of explaining the law of reflection 



tions for defective vision, namely convex by assuming that it was a property of light 



lenses for farsightedness and concave lenses always to take the shortest path between 



for shortsightedness. two points. Kepler tried to understand the 



In 1611 the Z)fop^nce was published, which path of the reflected ray from the specific 



is a "demonstration of those principles which resistance which the light finds in the dense 



relate to vision and visible objects on account media. 



of those glasses onl}^ lately invented." In Descartes, however, tried to establish a 



the preface the author states that the book complete system of optics on mechanical 



was inspired by the wonderful astronomical principles based on the assumption of an 



discoveries which Galilei had made with his ethereal fluid penetrating all bodies and the 



telescope and reported in the preceding year space around them extending throughout the 



1610. The book brings some new results in whole universe. The particles of this fluid 



all fields. The critical angle of total internal were thought to be very small and to act 



reflection is defined in the chapter dealing like balls bouncing off other bodies. In this 



with refraction. way Descartes explains the equality of the 



Kepler used the camera obscura for study- angles in reflection as a plausible mechanical 



ing experimentally the properties of some phenomenon. By suitable assumptions on 



lenses in order to confirm the results of his impact and resistance of those balls imping- 



mathematical considerations. Kepler arrived ing on the surface of optical media he arrived 



at the conclusion that the internal surface at the law of reflection which he properly 



of the crystalline eye lens must be a hyper- formulated as the constancy of the ratio of 



boloid for the best focusing conditions on the the sines of incident and reflected angles, 



retina, thus anticipating Descartes. Kepler Descartes does not mention the names of 



discussed combinations of convex and con- any other investigators in this connection, 



cave lenses both for each kind in itself and Usually the law of refraction is ascribed to 



with the other kind, which includes the Willibrord Snell, 1591-1626, a professor of 



Galilean telescope; what we call today the mathematics at Ley den University, who had 



astronomical or Keplerian telescope is de- taught about his law in his courses on optics, 



scribed. However, no reference to the use of a He had prepared a publication but it was 



combination of lenses as a microscope is never printed. Huygens is reported to have 



made, although the mathematical treatment seen the manuscript in which the law of re- 



of lens properties is quite thorough. It in- fraction was expressed in mathematical 



eludes for example also an investigation of terms, stating that for two different optical 



spherical aberration. media the ratio of the cosecants of the inci- 



Rene Descartes (1596-1650) published his dent and the refracted angles stays constant 



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