1876.] 
Sidereal Astronomy . 
time of Mayer, who formed a first catalogue of the double 
stars published in 1756. The philosophical astronomer 
Lambert, without having himself observed the double stars, 
published — in his “ Photometry ” (1760), and in his “ Cos- 
mological Letters ” (1761) — the first exaCt notions on the 
relations of mutual attraction which ought to exist between 
the components of these partial systems ; Lambert thought, 
with Kepler, that the distant suns ought to be surrounded 
—like our own sun — with a retinue of obscure stars similar 
to our planets and comets. As to the stars which were very 
near to one another, he believed — whilst favouring the idea 
of an obscure central body — that these stars ought to re- 
volve round their common centre of gravity, and to accom- 
plish their revolution in a sufficiently short space of time. 
John Michell, who did not know the ideas started by Kant 
and by Lambert, followed another course (1767). He 
applied the calculus of probabilities to the study of the 
stellar groups, and, above all, to the multiple stars. He 
proved that there were 500,000 chances to 1 that the union 
of these six principal stars of Pleiades could only be the 
effedl of accident, and that some one cause had been instru- 
mental in determining their approach. He himself became 
so persuaded of the existence of stars turning round each 
other, that he proposed the study of these partial systems 
as a means of solving certain astronomical problems. 
Christian Mayer, an astronomer of Manheim, has the 
great merit of having been the first who seriously observed 
the double stars (in 1778). This most important branch of 
his work was not acknowledged until some time after his 
death. 
These trifling but memorable beginnings were followed by 
the gigantic work of William Herschel, comprehending the 
long period of more than twenty-five years. The double 
nature of no star had actually been proved when William 
Herschel commenced his observations. The discovery of 
the true double stars is owing to his ingenious perseverance 
— above all to his observations of the beautiful double star 
Castor during a period of twenty-five years (1778 to 1803), 
showing that the relative positions of the two components 
varied, and that this variation had no relation with the 
annual translation of the earth. In following this study he 
was not long in proving many cases of really binary stars, 
groups in which the little star moved round the larger. 
“ Thus was proclaimed,” said Humboldt, “ this marvellous 
truth, that two luminous suns by themselves — and doubtless 
accompanied each with a planetary system — could turn 
VOL. vi. (n.s.) h 
