July 2, 1915] 



SCIENCE 



17 



there is a fainter star near it, which you 

 can see nicely on a fine night. These stars 

 are 600" apart ; with a big telescope with a 

 magnification of 600 times — and this is 

 about as high a magnification as can be 

 generally used in England — two stars 1" 

 apart are seen double just as clearly as 

 Alcor and Mizar are seen with the naked 

 eye. I think this is the most useful way to 

 think of 1" — a very small angle, which one 

 needs a magnification of 600 times to see 

 easily and clearly. Bradley showed that 

 y Draconis did not wander by this amount 

 from its mean position among the stars in 

 consequence of our changing viewpoint. 



The next attempt to which I wish to 

 refer is the one made by Sir William Her- 

 sehel. In a paper communicated by him to 

 the Eoyal Society in December, 1781, he 

 reviews the serious difficulties involved in 

 determining the parallax of a star by com- 

 paring its zenith distance at different times 

 of the year. Especially there is the uncer- 

 tainty introduced by the refraction of 

 light, and in addition as the angular dis- 

 tances of stars from the zenith are changed 

 by precession, nutation and aberration, any 

 errors in the calculated amount of these 

 changes will all affect the results. He pro- 

 posed, therefore, to examine with his big 

 telescope the bright stars and see which of 

 them had faint stars near them. The bright 

 stars, he said, are probably much nearer 

 than the faint stars; and if the parallax 

 does not even amount to 1" the case is by 

 no means desperate. "With a large telescope 

 of very great perfection it should be pos- 

 sible to detect changes in the angular dis- 

 tance of two neighboring stars. By this 

 differential method the difficulties inherent 

 in the method of zenith distances will be 

 eliminated. Herschel made a great survey 

 to find suitable stars, and in this way was 

 led to the discovery of double stars — i. e., of 

 pairs of stars which are physically con- 



nected and revolve around one another, just 

 like sun and earth. This was a most im- 

 portant discovery, but as the two compo- 

 nents of a double star are practically at 

 the same distance from us they do not serve 

 to determine parallax, for which we need 

 one star to serve as a distant mark. 



For another forty years persistent ef- 

 forts were made without success. Piazzi, 

 in Italy, thought he had detected parallax 

 in Sirius and a number of other bright 

 stars, but the changes he detected in the 

 zenith distances were unquestionably due 

 to errors introduced by uncertainty in re- 

 fraction, or slight changes in the position 

 of his instruments in the course of the 

 year. Dr. Brinkley, in Dublin, made a 

 gallant effort and took the greatest pains. 

 He thought he had succeeded, and for 

 many years there was a controversy be- 

 tween him and Pond as to ^v•hether his re- 

 sults were trustworthy. Tlie state of 

 knowledge of the distances of the fixed 

 stars in 1823 is summed up accurately by 

 Pond in the Philosophical Transactions: 



The history of annual parallax appears to me tO' 

 be this: in proportion as instruments have been 

 imperfect in their construction, they have misled 

 observers into the belief of the existence of sen- 

 sible parallax. This has happened in Italy to as- 

 tronomers of the 'very first reputation. The Dub- 

 lin instrument is superior to any of a similar con- 

 struction On the continent; and accordingly it 

 shows a much less parallax than the Italian as- 

 tronomers imagined they had detected. Conceiving 

 that I have established, beyond a doubt, that the 

 Greenwich 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. 



Besides these and other efforts to find 

 parallax in the zenith distances of stars, 

 attempts were also made to detect changes 

 in the time at which the stars cross the 

 meridian, to see if they are slightly before 

 their time at one period of the year and 

 slightly after it at another. But these, too. 



