May 11, 1900.] 



SCIENCE. 



723 



suits. It is now well established that, in 

 our traditional method of determining right 

 ascensions by transits of stars over the 

 threads of a meridian instrument, all ob- 

 servers, whatever method they adopt, note 

 the transit of a faint star too late relatively 

 to a bright one. The error varies with the 

 observer and the conditions to such an ex- 

 tent that its amount almost defies exact 

 determination. The problem of getting rid 

 of it is among the most serious with which 

 the astronomer has now to contend. 



One circumstance will illustrate the ex- 

 treme precision reached by Gill in this 

 work. The final discussion of his observa- 

 tions brought out a seeming periodic change 

 in the longitude of. Mars, going through its 

 period in about half a month. Theory 

 aiforded no explanation of such a change. 

 Years afterwards it was traced to an error in 

 the ephemeris of the planet, which had been 

 prepared for him by an official authority, in 

 which certain extremely minute terms of the 

 nutation were not consistently applied. So 

 minute an error would never have been de- 

 tected by any other method of observation. 



The result found for the solar parallax 

 was 8". 78. The main source of doubt to 

 which it is. subject, apart from unavoidable 

 accidental errors, arises from the possible 

 uncertainty in bringing the image of a star 

 into coincidence with the center of a plane- 

 tary disc. The effect of phase and a pos- 

 sible difference of personal equation in esti- 

 mating the center of the planet on the two 

 sides of the meridian introduce a certain 

 amount of doubt. At the same time there 

 is some reason to believe that the value is 

 as near the truth as any that we can get at 

 the present time. 



One result of this expedition is of more 

 than astronomical interest. It gave rise to 

 a very pleasant book by Mrs. Gill, ' Six 

 Months in Ascension,' describing life on a 

 far distant island within the tropics, as 

 well as the work of the expedition. 



In 1881 Gill was appointed Director of 

 the Observatory at the Cape of Good Hope, 

 succeeding Mr. E. J. Stone. Although 

 under the capable direction of the latter 

 this establishment had continued to add to 

 its reputation as one of the few important ob- 

 servatories in the southern hemisphere, no 

 one could have seen in it an inviting field 

 for astronomical research generally. Its 

 instruments were antiquated, its means in 

 other respects extremely limited, and its 

 staff insufficient to the execution of any im- 

 portant enterprise. The general impres- 

 sion might therefore have been that, while 

 an astronomer of ability might be able to 

 do some good work at the Cape Observatory 

 on traditional lines, it would be impossible 

 for him to enter upon a great career. But 

 the new astronomer soon showed that his 

 energies were not limited by his unfavor- 

 able surroundings. He had learned what 

 could be done with the heliometer, and 

 what further improvements could be made 

 in it in order to bring it to the highest state 

 of perfection. Then he found an inviting 

 field for its application. 



The question of the parallax of the fixed 

 stars has long been, from a scientific point 

 of view, one of the most important with 

 which the astronomer has to deal. To de- 

 termine this quantity is equivalent to de- 

 termining the distances of the stars and 

 hence making a step towards estimating 

 the dimensions of the universe and the ar- 

 rangement of its constituent bodies in space. 

 Twenty years ago the parallaxes of a num- 

 ber of fixed stars had been measured, but, 

 in the greater number of cases the diffi- 

 culty of the problem was such that the re- 

 sults were open to more or less doubt. The 

 new and perfected heliometer was the best 

 instrument for deciding the question and, 

 by using it in the southern hemisphere, it 

 was applied to a field almost entirely un- 

 cultivated. It is gratifying to us to know 

 that, in this work, he found an able col- 



