PRESIDEN'’S ADDRESS. 15 
Stellar Parallax. 
To extend exact measurement from our own solar system to that of 
other suns and other systems may be regarded as the supreme achieve- 
ment of practical astronomy. So great are the difficulties of the pro- 
blem, so minute the angles involved, that it is but in comparatively 
recent years that any approximate estimate could be formed of the true 
parallax of any fixed star. Bradley felt sure that if the star y Draconis 
had a parallax of 1” he would have detected it. Henderson by ‘the 
minute sifting of the numerical results’ of his own meridian observa- 
tions of a Centauri, made at the Cape of Good Hope in 1832-33, first 
obtained certain evidence of the measurable parallax of any fixed star. 
He was favoured in this discovery by the fact that the object he 
selected happened to be, so far as we yet know, the nearest sun to our 
own. Shortly afterwards Struve obtained evidence of a measurable 
parallax for a Lyre and Bessel for 61 Cygni. Astronomers hailed with 
delight this bursting of the constraints which our imperfect means im- 
posed on research. But for the great purposes of cosmical astronomy 
what we are chiefly concerned to know is not what is the parallax of 
this or that particular star, but rather what is the average parallax of a 
star having a particular magnitude and proper motion. The prospect of 
even an ultimate approximate attainment of this knowledge seemed 
remote. The star a Lyre is one of the brightest in the heavens ; the 
star 61 Cygni one that had the largest proper motion known at the time ; 
whilst a, Centauri is not only a very bright star, but it has also a large 
proper motion. The parallaxes of these stars must therefore in all 
probability be large compared with the parallax of the average star ; 
but yet to determine them with approximate accuracy long series of 
observations by the greatest astronomers and with the finest instruments 
of the day seemed necessary. 
Subsequently various astronomers investigated the parallaxes of other 
stars having large proper motions, but it was only in 1881, at the Cape of 
Good Hope, that general research on stellar parallax was instituted.! 
Subsequently at Yale and at the Cape of Good Hope the work was 
continued on cosmical lines with larger and improved heliometers.? By 
the introduction of the reversing prism and by other practical refinements 
the possibilities of systematic error were eliminated, and the accidental 
errors of observation reduced within very small limits. 
These researches brought to light the immense diversity in the absolute 
luminosity and velocity of motion of different stars. Take the following by 
way of example :— 
Our nearest neighbour amongst the stars, a, Centauri, has a parallax 
' Mem. R.A.S., vol. xviii, 
* Annals of the Cape Observatory, vol. viii. part 2, and Trans. Astron. Observatory 
of Yale University Yy, Vol. i. 
