xxxiv 



THK PKOGHKSS OF 



planet, which are six in number. They revolve 

 in a plane nearly per|KMi<li<-ular to the orbit of 

 the planet, ami contrary to the order of the 

 M^II.S. both these primary ami secondary bodies 

 obey, in their revolutions, the laws of Kepler. 

 The same remark applies to the satellites of 

 Saturn, which llcrsfhell increased to seven. 



6'. J)itcovcry of four new planets. Kepler was 

 of opinion that the harmony of our system wanted 

 a planet between .Mars and Jupiter. A similar 

 notion was entertained by Lambert, who thought 

 it likely that such a planet might exist, though 

 it was dark and invisible. The distances of the 

 planets from the sun may be denoted by the 

 series 4, 7, 10, 16, 28, 52, 100; which numbers 

 are convertible into English miles by multiplying 

 by 9.] millions. The dark or deficient planet 

 corresponds to twenty-eight, or a distance of 266 

 millions of miles from the sun. 



In the place assigned to this dark planet, four 

 very small bodies have been discovered ; which 

 some persons have conjectured to be only frag- 

 ments dissevered from the principal, and that 

 other similar fragments may still remain undis- 

 covered. The discovery of these very small 

 planets distinguished the commencement of the 

 present century. Piozzi discovered Ceres at 

 Palermo, on the 1st of January, 1801 ; Olbers 

 discovered Pallas at Bremen, on the 28th of 

 ."March, 1802; and Harding observed Juno on 

 the 2d of September, 1804, and Vesta on the 

 29th of March, 1807. These small bodies differ 

 from the other planets, not only by their diminu- 

 tive size, but by the remarkable inclination of 

 their orbits to the plane of the ecliptic ; which, 

 however, they intersect nearly in the same 

 nodes. 



7. Extent of the universe. Herschell, by 

 means of his excellent reflecting telescopes, em- 

 ployed himself assiduously in observing the 

 numerous clusters of nebulosities, and dis- 

 tinguished many of the changing and double 

 stars ; which, though suns of other systems, yet 

 appear connected, and may, probably, circulate 

 about their common centre of gravity. The sub- 

 ject has lately been carried a great deal further 

 by Sir James South and Sir John Herschell, the 

 latter of whom is preparing to go to the Cape of 

 Good Hope, to examine the double stars of the 

 southern hemisphere. 



Sir William Herschell assuming that the in- 

 strument which he used could enable him to 

 penetrate 497 times farther than Sirius, he 

 reckoned 116,000 stars to pass in a quarter of an 

 hour over the field of view, which subtended an 

 angle of only 15'. If we compute from such a 

 narrow zone, the whole celestial vault must dis- 

 play, within the range of telescopic vision, the 

 stupendous number of more than five billions of 



stars. If each of those be a sun to a system 

 similar to ours, and if the same number of planet* 

 revolve round it, then the whole planets in the 

 uni \erse will be more than fifty-live billions, not 

 reckoning the satellites, which may be much 

 more numerous. What an unimportant portion 

 of the universe is occupied by our earth, and 

 how insignificant must its inhabitants, with all 

 their cares, and pride, and vanity, appear to 

 that Almighty being whose providence regulates 

 and governs at once such a prodigious number of 

 worlds, with all their innumerable inhabitant^. 



OPTICS. 



THE history of astronomy, the most perfect 

 and the sublimest of all the sciences, has un- 

 avoidably extended itself to a considerable 

 length. Optics is the next science in point of 

 perfection. Like astronomy, it depends on ob- 

 servation, but is every where capable of the rigid 

 application of mathematical calculation. 



As light propagates itself in straight lines, it 

 seems to offer itself spontaneously to the con- 

 sideration of geometers. Euclid accordingly 

 began the investigation at least fifty years before 

 Archimedes had placed mechanics among the 

 number of the mathematical sciences. Euclid 

 only established two particulars : 1. That a point 

 in any object is seen in the direction of a straight 

 line from the eye to that object. 2. That when 

 a point in an object is seen by reflection from a 

 polished surface, the lines drawn from the eye 

 and from the object to the point whence the re- 

 flection is made, are equally inclined to the re- 

 flecting surface. From a treatise on optics by 

 Ptolemy, discovered some years ago in the king 

 of France's library, it appears that many ob- 

 servations had been made on refraction, though 

 the law followed had not been discovered. 



About 1000 years after Ptolemy, or in the 

 llth century, Alhazen wrote his treatise on 

 optics. In it he solved a problem of very great 

 difficulty, to find the point in a spherical specu- 

 lum, at which a ray coming from one given point 

 shall be reflected to another given point. Alhazen 

 was acquainted with the anatomy of the eye, 

 though he did not fully understand the uses of 

 the different parts. On seeing single with two 

 eyes, he made the very important remark, that 

 when corresponding parts of the retina are affect- 

 ed we perceive but one image. Roger Bacon, 

 who lived in the 13th century, made a near ap- 

 proach to the knowledge of lenses, and their use 

 in assisting vision. The date of the discovery of 

 spectacles, for assisting the sight, may be carried 

 back as far as 1313, but no farther. The in- 



