O PRESIDENT S ADDRESS. 



Africa — Henderson, Maclear^ Gill, Hough, each man a leader in 

 his own particular branch of enquiry. 



Henderson was not long at the Cape when the desire to 

 obtain a more accurate parallax of the moon, and, if possible, the 

 parallax of some of the stars, held him. 



His determination of the distance of Alpha Ccntauri is an 

 epoch in the history of astronomy. True, his results were pub- 

 lished after those of Bessel on 6i • Cygni, but the priority of 

 observations belongs certainly to Henderson. The delay of 

 almost seven years between observation and publication was due 

 to official neglect and indifference, neglect and indifference which 

 drove a man of Henderson's refinement and timidity into 

 reticence. But the work he did remains until this day. His 

 determination of the moon's distance, of the longitude and lati- 

 tude of the Cape, of the position of the principal southern stars, 

 can scarcely be bettered even with the most refined instruments 

 of to-day. He gave to South African astronomy a certain dis- 

 tinction and culture ; he breathed into it his own refinement and 

 scrupulous honesty. And yet his residence at the Cape is 

 measured by months, not years. What an elusive intangible 

 thing personality is. 



Henderson's work on stellar parallax was taken up by 

 Maclear. and carried to its furthest issues by Gill. The helio- 

 meter work of the latter, on certain well-known stars, will remain 

 the high-water mark in this direction until a new method of 

 determining stellar distances is discovered. 



But the name of Gill will be indissolubly associated with tlie 

 determination of the Sun's distance. I remember how, in 

 moments of confidence, seated by his study fire, he told how he 

 was attracted early to this problem, the problem of a generation 

 ago, just as stellar cosmography is the problem of to-day. When 

 Gill took up the problem, it was practically in chaos as a definite 

 singular deduction. 



Leverrier. even as late as 1872, despairing of ever determin- 

 ing the Solar parallax by any direct process, attempted to derive 

 it from some of its related functions, such as aberration and 

 nutation. 



The transit of Venus results of 1874 gave values that diff'ered 

 from one another by at least 3,000,000 miles, one-thirtieth of the 

 total .amount to be determined. It is difficult to realise that this 

 uncertainty belongs to a period of time only forty years remote 

 from our own. But lest the scientific worker in other fields 

 may judge ungenerously this amplitude of error, we may remind 

 ourselves that the total angle on which the determination of the 

 sun's distance depends is less than that subtended by a shilling at 

 a distance of one-fifth of a mile. Yet, to-day, we know the sun's 

 distance to one-thousandth part of its true value, to such a high 

 level of refinement has modern observations been brought. Gill's 



