OPTICS. 



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 lie 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 which 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, 

 he 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 wa'ter. See PTOLEMY. 



Alhazen, an Arabian writer, was the 

 jiext 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 Brahe. 

 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- 

 ferent 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 in the 

 horizon, than near the meridian. This 



VOL. V, 



phenomenon he ranks among 1 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 angle* 

 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- 

 gined 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 of the 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 if 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 retraction of rays 

 into it. 



In 1270, Vitellio, a native of Poland, 

 published a treatise on optics, containing 1 

 all that was valuable in Alhazen, and di- 

 gested in a belter manner. He observes, 

 that light is always lost by refraction, 

 which makes objects appear less luminous. 

 He gave a table of the results of his expe- 

 riments on tiie 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- 

 flection, to form the rainbow ; because 

 the body which the rays tall 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- 

 curacy. See REFRACTION. 



Contemporary with Vitellio was Roger 

 Bacon, a man of very extensive genius, 

 who wrote upon almost every branch of 

 science ; though it is thought his improve- 

 ments in optics were not carried tar be- 

 yond those of Alhazen and Vitellio : to 

 him, however, has been attributed the 

 invention of the MAGIC LANTERN, which 

 see. 



One of the next who distinguished him- 

 self in this way, was Maurolycua, teacher 



G 



