The next writer of any importance on the science of optics was Alhazen, an Arabian philosopher, who flourished in the century. He made many observations and ex periments on the effects of refraction at the surface between air and water, air and glass, end water and glass, from which he deduced that atmosphencal refraction increases the altitudes of all objects in the heavens. He also first observed that the stars are sometimes Isom above the horizon by means of refraction, when they are really below it; an observation couKitseff by Viti&io and other opticians. He likewise maintained that refraction con tracts the diameters and distances of heavenly bodies, and that it is the cause of the twink ling of the stars. Besides, Albania treats largely on the magnifying power of glasses, so that probably his observations led to the in vention of spectacles. In the next century followed Vitali°, a Pole, who digested the contents of Alliazen's work, and made many additional observations on the power of re fraction. He gave a table of the results of his experiments on the refracting power of air, water, and glass, corresponding to different angles of incidence. Roger Bacon, a con temporary with Vitali°, also wrote on this science, and is generally considered to be the inventor of the magic lantern. Manroly cue, who followed these two at the dktanee of nearly two centuries, explains, in his trea tise De Iamine et Umbra, the proms of vi sion, showing that the crystalline humour of the eye is a lens which collects the rays of light issuing from the objects, and throws them on the retina, where the fixes of each pencil is formed. From this principle he dis covered the reason why some people have a short sight and others a long one; also, why the former are assisted by concave glasses, and the latter by convex ones. John Baptista
Porta, his contemporary, discovered the came ra onxura, and took the first public notice of the magic lantern, the original invention of which has been ascribed to Roger Bacon. Kircher, who followed Baptista, enlarged on his hints, and put them into execution. He also made many experiments with the camera obscure, by which he satisfied himself that vision is performed by the intromission of something into the eye, and not by visual rays, proceeding from it, as had been formerly, imagined. He considered the eye as a camera, obscura, the pupil to be the bole in the win dow shutter, and the crystalline humour to correspond to the wall which receives the images; hot in this latter point his idea has been proved, by closer observations, to be in correct, for it is now known that this office is performed by the retina. The observations and experiments of this writer on the science of optics, and on the nature of Tidal, to have led the way to the discovery scopes, which was doubtless made very soon after his time After ems, the writers on optics became very n erons, and their laboors contributed to the confirmation and improvement of those who had preceded them.. ,Among the works entitled to particu lar notice are Barrow's Optical lectures, Huygen's Dioptrics, Hartsoeker's Essaie Dioptrique, David Gregory's Elements of Dioptrics and Catoptrics, Dr. Smith's Optics, Wolfius's Dioptrics and estop:ries, Harris's Optics, but above all, Newton's Treatise on Optics, and his Optical Lectures.