It appears also, that the ancients were not unacquainted with the magnifying power of glass globes filled with water, though they probably knew nothing of the reason of this power ; and it is sup posed that the ancient engravers made use of a glass globe filled with water to Magnify their figures, that they might Work to more advantage.
Ptolemy, about the middle of the se cond century, wrote a considerable trea tise on optics. The work is lost ; but from the accounts of others it appears that he there treated of astronomical re fractions. The first astronomers were not aware that the intervals between stars appear less when near the horizon than in the meridian ; and on this account they must have been much embarrassed in their observations ; but it is evident that Ptolemy was aware of this circumstance by the caution Mitch he gives to allow something for 'it, whenever recourse is had to ancient observations. This philo sopher also advances a, very remote hy pothesis, to account for the remarkably great apparent size of the sun and moon when seen near the horizon. The mind, lie says, judges of the size of objects by means of a preconceived idea of their distance from us ; and this distance is fancied to be greater when a number of objects are interposed between the eye and the body we are viewing, which is the case when we see the heavenly bo dies near the horizon. In his Almagest, however, he ascribes this appearance to a refraction of the rays by vapours, which actually enlarge the angle under which the luminaries appear, just as the angle is enlarged by which an object is seen from under water. See PTOLEXY.
Alhazen, an Arabian writer, was the ,next author of consequence, who wrote about the year 1100. Alhazen made many experiments on refraction, at the surface between air and water, air and glass, and water and glass ; and hence he deduced several properties of atmosphe rical refraction, such as, that it increases the altitudes of all objects in the heavens; and he first advanced that the stars are sometimes seen above the horizon by means of refraction, when they are really below it ; which observation was con firmed by Vitellio, Walther, and especial ly by the observations of Tycho Ilrahc. Alhazen observed, that refraction con tracts the diameters and distances of the heavenly bodies, and that it is the cause of the twinkling of the stars. This re fractive power he ascribed, not to the va pours contained in the air, but to its'dif fereut degrees of transparency. And it
was his opinion, that so far from being the Cause of the heavenly bodies appear ing larger near the horizon, it would make them appear less; observing that two stars appear nearer together m the horizon, than near the meridian. This phenomenon he ranks among Optical de ceptions. We judge of distance, he says, by comparing the angle under which objects appear, with their sup posed distance ; so that if these angles be nearly equal, and the distance of one object be conceived greater than that of the other, this will be imagined to be the larger. And he further observes, that the sky near the horizon is always ima glued to be further from us than any other part of the concave surface.
In the writings of Alhazen, too, we find the first distinct account ofthe magnifying power of glasses, and it is not improbable that his writings on this head gave rise to the useful invention of spectacles ; for he says, that it' an object be applied close to the base of the larger segment of a sphere of glass, it will appear magnified. He also treats of the appearance of an object through a globe, and says that he was the first who observed the refraction of rays into it.
In 1270, Vitellio, a native of Poland, published a treatise on optics, containing all that was valuable in Alhazen, and di gested in a better manner. lie observes, that light is always lost by retraction, which makes objects appear less luminous, He gave a table of the results of his exile. riments on the refractive powers of air, water, and glass, corresponding to differ ent angles of incidence. He ascribes the twinkling of the stars to the motion of the air in which the light is refracted ; and he illustrates this hypothesis by observing, that they twinkle still more when viewed in water put in motion. He also shows, that refraction is necessary as well as re• fiection, to form the rainbow ; because the body which the rays fall upon is a transparent substance, at the surface of which one part of the light is always re flected, and another refracted. And he makes some ingenious attempts to explain refraction, or to ascertain the law of it. He also considers the foci of glass spheres, and the apparent size of objects seen through them, though with but little ac.