48 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



mines 

 direction 

 of motion. 



Motion of to be the real displacement of our sun and system in 



the solar space. In 1760 Mayer gave a list of 80 stars whose 



system in prO p er mo tions he determined by a comparison of 



SJadley- Homer's observations. He conceived the possibility 



Mayer. of determining the direction of the solar motion by 



the changing aspect of the constellations, keeping in 



view, however, that the effect is complicated by real 



independent movements of each individual star. 



Lambert and Mitchell entertained similar views, but 



carried them to no practical result. 



(211.) Here, then, Herschel appears on the field. He 

 Sil> Y -Her " proceeded in 1783 to do what his predecessors had 

 the discussed the possibility of doing. His own observa- 

 tions were not available in a problem like this. Tak- 

 j n g fa Q ^st values he could obtain of the Proper 

 Motions of only about 20 stars, he, with great saga- 

 city and success, divined that the solar system is 

 moving somewhere in the direction of X Herculis, a 

 point in the heavens whose right ascension is 257, 

 and north declination 25. This result he arrived at, 

 almost without calculation, by a mere consideration 

 of the changing perspective of a multitude of ob- 

 jects amongst which the spectator moves. Mayer had 

 compared the change to that seen when we walk 

 slowly through a grove of rather distant trees. Those 

 in front will seem to widen or expand as we approach, 

 those behind will condense towards the point we are 

 quitting. The greatest proper motions will be in the 

 objects on our right and left as we advance. It is 

 not to be supposed that we find in the stars the exact 

 counterpart of this change of place. For each star 

 may by its own absolute movement in space either 

 conspire with or oppose the motion due to perspec- 

 tive. But a combination of the whole results will 

 make apparent the direction of that part of the motion 

 due to a common cause, that is, to the translation of 

 our system in space. A cleverer approximation than 

 Herschel's was never made. He returned to the sub- 

 ject in 1805, but there is reason to think that his 

 first result was the more correct. 



(212.) As nothing essential has been added to Herschel's 

 More recent disco very of the direction of the solar motion, we 

 researches ghaJ] } iere re f er once f or a ll to the important con/ir- 

 on the same , . , . , . j T> r- A 



subject. mations which it has since received. Professor Ar- 



gelander, availing himself in 1835 of the improved 

 state of astronomy since Bradley's observations were 

 made, and since their reduction by Bessel, considered 

 the whole problem in a general and geometrical 

 manner, including every well-determined proper mo- 

 tion 1 by means of appropriate equations of condition, 

 which, being resolved by the method of least squares, 



give the direction of solar motion 261 11' of right 

 ascension, and 30 58' of north declination, differing 

 respectively about 4 and 6 from Herschel's first 

 numbers. Perhaps a still more convincing confirma- 

 tion was obtained by Mr Galloway (Phil. Trans. 1847), 

 from the proper motions of stars of the southern he- 

 misphere alone, which lead to a result nearly coin- 

 ciding with the above. 



Sir William Herschel's leading discoveries in Side- (213.) 

 real Astronomy may therefore be reduced to these Summary 

 the discovery of binary systems of stars and the orbits sc h e l's di 

 of several revolving stars ; the discovery and classifi- coveries. 

 cation of a prodigious multitude of nebulas ; the law of 

 grouping of the entire visible firmament, and its con- 

 nection with the great nebula of the Milky Way; and 

 lastly, the determination of the fact of the motion of 

 our sun and system in space, and the direction of 

 that motion. Setting aside all that is valuable, in- 

 genious, and noble in his farther speculations, and 

 all that he contributed to the enlargement of our 

 knowledge of the system of sun and planets with 

 which we are more immediately connected, these po- 

 sitive discoveries will ever remain a magnificent tro- 

 phy of his perseverance and success as a natural phi- 

 losopher. 2 



When his increasing years rendered the relaxation ( 214 -) 



of his arduous course of telescopic observation ad- , 8 ? ys 

 i ' -n i i i n ca * obser 



visable, whilst all his other faculties were in the vations. 



highest vigour, he began to devote more attention to 

 physical enquiries less directly connected with astro- 

 nomy. The nature of the emanations of light and heat 

 proceeding from the sun was a matter on which he 

 was forced to speculate in connection with his ideas 

 of the sun's constitution ; and even the practical en- 

 quiry as to the kind of dark glasses best fitted for 

 defending the eye in observations of that nature, led 

 him, in conformity with his usual habit, to a large 

 series of experiments on solar heat, and its trans- 

 missibility through glasses of different colours, and 

 other bodies. In the progress of these he was led Hl im % 

 to examine the heating power of the different rays o/ S pe C t runi 

 the spectrum, and consequently the refrangibility of 

 heat. He then arrived at the real discovery that the 

 place of greatest heat in a spectrum formed from the 

 sun's rays by a flint-glass prism is considerably be- 

 yond the extreme visible red ray. But we shall re- 

 turn to this result in connection with the history of 

 radiant heat. He also made some experiments on 



1 A small star in Ursa Major (1830 of Groombridge's Catalogue) has an annual proper motion of 6"'97 ; the star 2151 Pup- 

 pis, is the greatest known, being 7"'87. 



2 K"o historian of science, or biographer of Sir W. Herschel, can fail to acknowledge with gratitude the important assistance 

 afforded by MM. Arago and Struve to a compendious acquaintance with his writings and discoveries; the former by his article on 

 Herschel in the Annuaire du Bureau des Longitudes for 1842, the latter in his Etudes d' Astronomic Stellaire. While acknowledg- 

 ing my obligations to these works, I shall not be suspected by any careful reader of having dispensed with a reference to the 

 original memoirs ; still less of having adopted without examination the conclusions of these eminent authors, from whom, indeed, 

 on some important points, I differ entirely. 



