The Arabian philosopher must be considered as hav ing made a great step in the science, when he refuted the opinions concerning vision, which had been so long established. He proves that the rays which produce vi sion, come from the object to the eye. He gives an ana tomical description of the eye, explains the share which each of its parts has in vision, and accounts for single vision with two eyes, by supposing that, when two cor responding parts of the retina are affected, the mind per ceives but one image. Alhazen describes seven species of mirrors, whereas Ptolemy had considered only three ; and he has the sole merit of determining in general the focus of reflected rays, when the place of the eye and that of the object are known,—an honour which Alontucla, without evidence, supposes to belong to Pto lemy. Alhazett's instillment for measuring retraction is much more complex than that of Ptolemy ; and all that he says, both on the•refraction of fluids and on that of the atmosphere4 is much less perfect. Affiazen ob served that refraction contracts the diameters and dis tances of the heavenly bodies ; and he supposes it to be the cause of the twinkling of the stars. Ile treats fully of optical deceptions as produced by direct vision, as well as in vision by refracted and reflected light.
In the year 1270, Vitello, a native of Poland, drew up a very volutninous work on optics, which, though less prolix and more methodical than that of Alhazen, has in general been considered as little more than a translation of it. This, however, is too harsh a censure upon the Polish philosopher, though at the same time it may be considered as a just punishment for his having denied that he had any knowledge of the work of Alhazen.t His experiments on the refraction of light by water and glass, are much more correct than those of Ptolemy; and it is strange that the comparison of his own results did not lead him to the discovery of the constant ratio of the sines of the angles of incidence and refraction. The following Tables contain the results of all his experi MentS.
From these Tables it appears that the average mean ratio of the sines, both in the refraction of water and glass, is more correct than that of Ptolemy. Thus Water. Glass.
Ptolemy 0 76736 0.68736 Vitello 0.76414 0.66976 Newton 0.74858 0.64516 and the correctness of the individual results, when com pared with those of Ptolemy, will be manifest, by com paring the ratio of the sines for each angle with the mean ratio of the whole. In his experiments on the refraction of light, in passing from a dense into a rare medium, he found that the angles were exactly the same as when the light passed from a rare into a dense medium ; and upon this principle lie seems to have computed the refractions in the last row of Tables.
Vitello observed, that wherever light suffered refrac tion or reflexion, a certain part of it was lost, and the bodies alv,.ays appeared less luminous ; but he does not attempt to estimate the quantity of it. He ascribes the twinkling of the stars to the motion of the air in which the light is refracted ; and in an ex pet imental illustration of this opinion, he observes that the twinkling is increas ed when the stais are viewcd through water put in mo tion. In treating of the rainbow, he proves that refrac tion, as well as refiexion, is necessary to its production; but he seems to consider the refraction not as producing the colours, but merely as serving to condense the light, and enable it to make a stronger impression on the cye. The three colours which he supposes to exist in the rainbow, he ascribes to a mixture of the sun's light with the blackness of the cloud from which they ale reflect ed. He endeavours to show, that in some countries a rainbow cant,ot be seen at mid-day, viz. when the sun is higher than the semidiameter of the bow, the centres of the sun and the bow being always in a right line, passing through the eye ol the observer. In order to exhibit colours similar to those of the rainbow, he laid a piece of white paper under a circular glass vessel filled with water, and, when it was exposed to the sun, he saw co lours which he considered as analogous to those of the rainbow. The foci of spheres of glass, and the apparent size of objects seen thiough them, arc also among the topics discussed hy the Polish optician. Consideting that the focus of rays falling perpendicularly upon every point of the sphere must be in its centre, Ile supposes that this must also be the focus of the solar rays ; and as it was impracticable to determine the truth of this supposition experimentally, he suggested that a hemi sphere, or a still less portion of a sphere, might be em ployed to make the experiment. This mistake of Vi tello, in confounding the focus of rays converging to the centre of the sphere with the focus of parallel rays, has been censured by Baptista Porta ; but the learned Ita lian should at the same time have stated, that Vitello demonstrates 'the possibility of firing bodies placed be yond the sphere by the transmitted rays of the sun."' About ten years after the appearance of Vitello's op tics, Thomas Peckham, Archbishop of Canterbury, pub lished a treatise on optics entitled PeraPectiva Cone munis, or a treatise on direct optics, accompanied with an abridgment of catoptrics. This work, which was long considered as classical, went through many editions, though it contained nothing that was either very new or very important.