170 EICHELBERGER : DISTANCES OF HEAVENLY BODIES 



ration of light, though best known of all, may also be liable to some 

 small errors, since the observations from which it was deduced laboured 

 under all the foregoing difficulties. I do not mean to say, that our 

 theories of all these causes of error are defective; on the contrary, I 

 grant that we are for most astronomical purposes sufficiently furnished 

 with excellent tables to correct our observations from the above men- 

 tioned errors. But when we are upon so delicate a point as the parallax 

 of the stars; when we are investigating angles that may, perhaps, not 

 amount to a single second, we must endeavour to keep clear of every 

 possibility of being involved in uncertainties; even the hundredth 

 part of a second becomes a quantity to be taken into consideration. 



Herschel then proceeds to advocate selecting pairs of stars 

 of very unequal magnitude and whose distance apart is less than 

 five seconds, and making very accurate micrometric measures 

 of this distance from time to time. The first condition, should 

 give, in general, stars very unequally distant from the Earth, 

 so that the changing perspective as the Earth revolves in her 

 orbit would give a variation of the apparent distance between 

 the stars, while the small distance, less than five seconds, would 

 eliminate from consideration entirely any effect upon this dis- 

 tance of the uncertainties in refraction, precession, nutation, 

 aberration, etc. Herschel had already commenced the catalogu- 

 ing of such double stars and in January, 1782, submitted to the 

 Royal Society a catalogue of 269. This work did not enable 

 Herschel to determine the distances of the stars but did enable 

 him to demonstrate that there exist pairs of stars in which the 

 two components revolve the one around the other. In twenty 

 years he had found fifty such pairs. 



Coming forward another generation, that is, to a time a little 

 less than a hundred years ago, we find Pond, then Astronomer 

 Royal, writing: 



The history of annual parallax appears to me to be this: in pro- 

 portion as instruments have been imperfect in their construction, 

 they have misled observers into the belief of the existence of sensible 

 parallax. This has happened in Italy to astronomers of the very first 

 reputation. The Dublin instrument is superior to any of a similar 

 construction on the continent; and accordingly it shows a much less 

 parallax than the Italian astronomers imagined they had detected. 

 Conceiving that I have established, beyond a doubt, that the Green- 

 wich instrument approaches still nearer to perfection, I can come to no 

 other conclusion than that this is the reason why it discovers no parallax 

 at all. 



