846 
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 
pecs ‘2 proper motions he determined by a comparison of 
eadiay <= Rémer’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. 
11.) Here, then, Herschel appears on the field. He 
Bir W Her” proceeded in 1783 to do what his predecessors had 
mines the discussed the possibility of doing. His own observa- 
direction tions were not available in a problem like this, Tak- 
of motion. ing the best 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 A 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 eact 
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 
Morerecentdiscovery of the direction of the solar motion, we 
shepard here refer once for all to the important conjir- 
subject. | ™ations 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 by means of appropriate equations of condition, 
which, being resolved by the method of least squares, 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
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— S°™™#"Y 
the discovery of binary systems of stars and the orbits Cchel's aise 
of several revolving stars ; the discovery and classifi- coveries. 
cation of a prodigious multitude of nebule; 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.? 
When his increasing years rendered the relaxation _ (214.)_ 
of his arduous course of telescopic observation ad- my Beach 
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 omer fe 
to examine the heating power of the different rays of chad: 
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 
2 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 No historian of science, or biographer of Sir W. Herschel, can fail to acknowledge with gratitude the important assistance 
afforded by MM. Arago and Struve toa 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 Ztudes d’ Astronomie 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. 
