Double Stars. 137 



If we look s from a Ophiuchi, a little f, we shall come upon 

 /3 Ophiuchi, 3 mag., the brightest of its neighbourhood, lying 

 from a as far again as the distance between the " two heads." 

 /3 has 7, 4 mag., a little way sf, and 7 is followed by a vertical 

 line of three small stars, with a fourth nearly following the cen- 

 tral one, but somewhat to the s. The central one of these three 

 is — 



41. 67 Ophiuchi. 54"- 7. 143 0, 6. 4 and 8. Straw colour 

 and purple. Wide, but a pleasing contrast in size and colour, 

 in a fine field of minute stars. 



The fourth star, nearly following the central one, is — 



42. 70 Ophiuchi (often designated "p"). 5""43. 136°-4 

 (1830-76). 6"-8. 119°-7 (1847-48). 4| and 7. Topaz yellow and 

 purplish. This is one of the most celebrated binary systems, 

 and great trouble has been taken with it by many of the first 

 observers. Madler fancied its movements could not be reconciled 

 with the law of gravity. Strove found that his Dorpat tele- 

 scope gave smaller measures than his former ones, or than those 

 of Sir J. Herschel, South, and Dawes. Jacob, after paying 

 much attention to this object, is still dissatisfied, and suspects 

 disturbance from a third invisible companion. Secchi, again, 

 refers such discrepancies to errors of observation. In fact, 

 where the quantities to be ascertained are so extremely minute, 

 it is evident that there must be unavoidable differences 

 arising not only from the imperfection of instruments, but from 

 the diversity of eyes and judgments. The latter source of disa- 

 greement, known among astronomers by the term ' ' personal 

 equation/' has a wide range of influence, and has been made 

 the subject of considerable inquiry ; since with the same micro- 

 meter some astronomers are known to measure distances very 

 differently from others ; and the judgment of the same eye 

 seems liable to change ; and all this requires to be allowed for 

 in comparisons. But in such hair-splitting processes the gene- 

 ral agreement which will be found to obtain between really 

 good observers is much more remarkable than their occasional 

 discordance. In the present instance there can be no question 

 as to the existence or the shortness of the period ; but it has 

 been variously given between 112 years (Jacob) and 74 years 

 (Encke). Smyth and Sir J. Herschel prefer about 80 years. 

 Secchi's later measures, 1860-638, gave 6"-022 and 105-46. The 

 distance is, undoubtedly, again on the decrease. This pair 

 acquires an extraordinary interest from the recent determination 

 of its parallax — 0"*169 — byKriiger, Argelander's assistant at 

 Bonn. This, of course, though he considers its limits of error 

 to be only 0" - 0103 plus or minus, is open to revision by other 

 instruments ; but adopting it as a basis till disproved, it will 

 give us a distance which light could only traverse in 19^ years, 



