216 HISTORY OF SCIENCE. 



1716 that Halley drew the attention of astronomers to this elegant 

 and accurate method ; but as the next transit of Venus was calculated 

 for 1761, he could not, being then sixty years of age, expect to live 

 himself to witness it ; but he exhorted future astronomers not to allow 

 the opportunity to pass without uniting their efforts to obtain the best 

 observations of this rare astronomical occurrence. 



Halley devoted much attention to the moon's motions, and his im- 

 provements in the lunar tables were of great practical importance in 

 the problem of finding the longitude. He detected the very slow but 

 continuous acceleration of the moon's velocity, which is known as the 

 secular acceleration. It was an additional glory for Halley to be the 

 first who predicted the return of a comet. He observed that several 

 successive appearances were recorded which presented certain ele- 

 ments in common, and he concluded that these might be reappear- 

 ances of the same body travelling in an elliptic orbit. Newton had 

 before in his "Principia " boldly assumed that comets, like planets, 

 revolve about the sun. The comet of 1682 was identified by Halley 

 with those of 1607 and 1531, and he predicted that it would return about 

 the end of 1758 or the beginning of 1759. It was observed on the 

 25th of December, 1758, and was a conspicuous object in the heavens 

 in the spring of 1759. It reappeared in 1836, and will not again be 

 seen until 1912. Historical records of the appearances of great comets 

 agree with the period of Halley's comet, and have been traced back 

 to ii B.C. 



After the publication of the " Optics " of Descartes, this branch of 

 science remained for a number of years without any remarkable addi- 

 tion, unless we reckon such inventions as that of the Magic Lantern 

 by Kircher, 1646. But during the seventeenth century a remarkable 

 and important series of discoveries completely changed the aspect of 

 the science of light. JAMES GREGORY, of Aberdeen (1639 1675), an 

 excellent mathematician, conceived the idea of the Reflecting Tele- 

 scope in 1663, and the following year he visited London in order to get 

 a speculum constructed, but he could find no artist capable of under- 

 taking the task of grinding a speculum of a parabolic form, and he 

 was obliged to renounce the execution of his project for the time. 

 Gregory published a " Treatise on Optics,''' containing many valuable 

 observations on optical instruments, particularly telescopes. Although 

 Gregory failed in having his telescope carried into execution, on ac- 

 count of seeking for the perfect theoretical form for the mirror, the 

 instrument as designed by him deserves notice as the first idea of the 

 reflecting telescope. It is represented in section in Fig. 102, where 

 o is the aperture of the tube, a b a concave mirror which reflects the 

 incident rays back to the small concave mirror at N, whence they are 

 again reflected towards a small circular opening in the centre of the 

 principal mirror, in which is placed the eye-glass M p. Cassegrain, 

 a Frenchman, modified this arrangement by substituting a convex 



