An Introductory Treatise on the Nature and Properties of Light, and on Optical Instruments (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1829 Excerpt: ...after this time Snellius discovered the method of measuring refraction by means of the sines. Many have believed that Des Cartes was the first inventor, as it appears in his works, but Huygens declares, on his own knowledge, that Des Cartes had transcribed it from the papers of Snellius. Des Cartes explained optical refraction on the principle of the mechanical resolution of forces. Dr. Halley particularly honours this great man when he says, that, "although some of the ancients mention refraction as the effect of a transparent medium, yet Des Cartes was the first who discovered the laws of refraction, and reduced Dioptrics to a science. To determine the value of the refraction of water the Royal Society, in 1664, made an experiment, the result of which was, that if the angle of incidence was 40, the angle of refraction was about 30; that the refraction of salt water was greater than that of fresh; and that a solution of salt petre was a little more, and a solution of alum a little less refractive than common water. From these and other experiments, they discovered that the refraction of any medium is not in proportion to their density. In 1708 the Royal Society made experiments to determine the refractive power of atmospheric air. When the barometer was at 29. 7, and thermometer 60, they found the sine of incidence in vacuo, to the sine of refraction in common air as 1.000.000 to 999.736, which was afterwards confirmed by the Academicians of Paris. A short time before this, Newton had made his great discovery concerning the different refrangibility of the rays of light, and Bartolinus concerning double refraction. But as these subjects form a distinct part of optics, their history is deferred. THE THEORY OF VISION. CHAPTER I. On the Anatomy of the Eye...