176 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1916. 



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 5", would eliminate from consideration entirely any effect 

 upon this distance of the uncertainties in refraction, precession, 

 nutation, aberration, etc. Herschel had already commenced the 

 cataloguing 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 20 years he had 

 found 50 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 pax*allax appears to me to be this: In proportion as 

 instruments have been imperfect in their construction they have misled ob- 

 servers 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. 



Within 15 years after this statement by Pond observations had 

 been obtained which showed a measurable parallax of three different 

 stars. The announcements of these results, each by a different 

 astronomer, were practically simultaneous. 



W. Striive, using a filar micrometer, determined the distance of 

 a Lyrae from a small star about 40" distant on 60 different days 

 over a period of nearly three years. He obtained a parallax of 

 0".262±:0'^025. Bessel, using his heliometer, determined the dis- 

 tances of 61 Cygni from two small stars distant about 500" and 700", 

 respectively. He obtained for this star a parallax of 0".314d::0'".020. 

 Henderson, using determinations of the position of a Centauri by 

 meridian instruments, deduced a parallax of l".16±0".ll. All 

 three of these results were announced in the winter of 1838-39 and 

 indicate that the three stars are distant from the earth about 750,000, 

 650,000, and 200,000 times the distance of the sun from the earth. 



