Optics During the Seventeenth Century

The growth of experimental science during the sixteenth and seventeenth centuries gave an impulse to the study of the primary vehicle of all observations-light-and to the development of the instruments extending the observing power of the human eye. The Renaissance artists had investigated optical questions in order to obtain naturalistic representations and to improve the perspective in their paintings. Then viewing instruments more powerful than the existing spectacles and magnifying glasses were sought for, a movement which culminated with the invention of the telescope and the compound microscope by the spectacle makers of Middleburg in Holland, Hans Lippershey and Zacharias Jansen, about the beginning of the seventeenth century. The scholars of the time-notably Galileo and Kepler-took up these craft discoveries and studied the theoretical principles which they embodied. During the seventeenth century attention was concentrated on the telescope, for it was of great use in astronomy and navigation, and its defects were less serious than those of the early microscope. However, the microscope was made and used by the scientists of the time. Galileo studied the anatomical structure insects with the microscope about 1610, and his work was continued in England by Robert Hooke in the 1660’s, whilst towards the end of the seventeenth century a flourishing school of microscopists developed in Hollad.

Galileo did not add much either to the theory or to the intruments of optical science. In principle his telescope was the same as that of the Dutch spectacle makers, consisting of a convex and a concave lens, though he improved the performance of the instrument. In contrast Kepler designed several new telescope, notably the astronomical telescope with two convex lenses, and he founded modern experimental mechanics, and Gilbert the modern science of magnetism. Kepler formulated intuitively the inverse square law of the diminution of light intensity with distance from the consideration that light radiated spherically from a given source. Studying the bending of a light beam at an interface between two transparent media, Kepler showed that Ptolemy’s approximate law of refraction, which supposed a direct proportionality between the angles of incidence and refraction, was true only for angles less than anout 30°. He thought that refractive power of a meddium was proportional to its density, but the English mathematican, Harriot, pointed out to him that oil is more refractive than water, though less dense.

The correct law for the refraction of light was discovered in 1621 by Willebrod Snell, 1591-1626, a professor of mathematics at Leiden, who found that the sines of the angles of incidence and refraction always bore the same ratio one to the other for a given interface between two media, the ratio being termed the refractive index for that interface. This law of refraction was first made known in 1637 by Descartes, who endeavoured to explain it and other optical phenomena, on the supposition that light consisted of small particles in rapid linear motion. He held that the reflection of light was just a rebound of the light particles from an elastic surface according to the laws of mechanics. Similarly, the refraction of light on passing from a dense to a light medium was analogous to a ball breaking through  a thin cloth. The component of the ball’s velocity at right angles to the cloth was reduced by the resistance of the cloth, but the velocity component parallel to the cloth remain unchanged. Hence the over-all velocity of the ball would be decreased and its path would be bent towards the passed from a dense to a light medium. The analogy implied that light travelled faster in dense than in light media. Such a consequence we may well understand, said Descartes, if we remember that a ball rolls more easily along a hard, dense table than across a soft, light carpet.

Descartes had a second theory of light, according to which light was an action or pressure transmitted from an object to the eye through the closely packed matter of the intervening space. He suggested that light was like the pressure transmitted from an object to the hand of a blind man through his stick. Descartes believed that it was the pressure of light from the sun which maintained the vortex of the solar system rigid against the pressure of the vortices of the stars outside of it. Thus the centrifugal force of the cosmic vortices was nothing other than the pressure of light from their central regions. The different colours of light were produced by the different speeds of rotation of the matter in space, red was produced by the fastest motion, and blue by the slowest. The theory that light was an action transmitted by the ether of space was developed by Cartesians, whilst the corpuscular theory of light was taken up by Newton and his followers.