IJGIIT (OPTICAL) MICROSCOPY 



if the angle of incidence varies. Later, also the back through a compound microscope 



by Descartes, this law was expressed by the the tube of which is put through a hole in 



respective sine functions. the vertex of the mirror. An interesting fea- 



The publication of Descartes' Dioptriqiie ture of this scheme is Descartes' proposition 



was followed by a violent controversy which to use a Galilean type arrangement of one 



continued even after Descartes' death. The convex and one concave lens, both of which 



theoretical derivation of the law was assailed, have to be hyperboloid. 



Descartes had stated that the velocity of Still another proposition of Descartes has 



light in the denser medium was increased, the purpose of diminishing spherical aberra- 



whereas his opponents argued that it would tion in a unique way. He suggested putting 



be retarded. Hero had argued for the law of a water-filled tube with a membrane at one 



reflection that a ray would follow the short- end right at the eyeball. The membrane 



est path. For the law of refraction, Fermat would then adapt itself and its shape to the 



assumed that a ray following an intrinsic contour of the eyeball. The other end of the 



law of nature would travel that path in a tube would be closed with a curved glass disc 



denser medium which requires the shortest shaped similar to the eye curve. The effect 



time. For this argument it must be assumed would be to increase the depth of the aqueous 



then that the velocity of light would be re- humour of the eye because the refractive 



duced in a denser medium. In the eighth indices of this Hquid and of water are about 



chapter of the Dioptrique Descartes showed the same. From such an arrangement Des- 



how the aberration of a spherical lens could cartes expected larger images of the objects 



be avoided by non-spherical surfaces as for observed, but he realized the inconvenience 



example ellipsoids or hyperboloids. He gave in practical use. He suggested replacing the 



detailed instructions how to grind and polish water-filled tube by a long glass cylinder 



this kind of lens and found one or two glass with a suitably curved surface at both ends, 



makers who tried to make them, but without or a hollow tube with lenses at both ends, 



much success. Their surfaces had to be shaped in a special 



Descartes suggested also an application of way as required by his idea of increasing the 



nonspherical lenses for single lens micro- effective depth of the human eye humour by 



scopes. One lens was to be mounted in the optical means. 



vertex of a spherical concave mirror so Athanasius Kircher, lGOl-1680, wrote the 



that it would protrude a little through a book Ars Magna Lucis et Umbrae the first 



hole in the mirror, the object being held at edition of which was published in Rome 



its focus by means of a prong. The whole 1646 and the second edition in Amsterdam 



apparatus had to be held close to the eye and 1671. It contains one section "De Mira 



aimed at the sun. The light rays would illu- Rerum Naturalium Constitutione per Smi- 



minate the object and it would appear croscopium Investiganda." The word Smi- 



greatly magnified if observed through the croscopium used here comes from the Ionian 



lens. This instrument was intended for hand dialect in which Herodotus wrote. Kircher is 



use only. the only author using this word. It fell out 



Descartes described another more refined of use in later years. In his book he gives an 



one to be mounted on a high stand in the account of all the microscopes of his day: 



same way as telescopes are mounted. A para- (1) Two convex lenses are used, 



bolic mirror is provided, in the focus of (2) Glass spheres filled with water are 



which the object has to be held. The illumi- used as single lenses. 



nation is enhanced by means of a convex lens (3) Tiny glass spheres at the end of a tube 



in front of the object. It is observed from are used with the object to be put right on it. 



456 



