210 Transactions of the Royal Society of South Africa. 
practically no parallax — that is, they were at an infinite distance — he was 
assured in his own mind that a general uniformity was not the order of 
space. This, however, did not hinder or obscure the carrying out of his 
scheme of parallax determinations, for he knew that when this theory and 
that would be urged in explanation of the structure and movements of 
the universe of stars, they would have to be tested finally by definite 
parallax and proper motion results. 
The data that he secured in this direction have proved of great value in 
the investigations of star streaming instituted by Kapteyn and carried on 
by Eddington, Dyson, Swarzchild, and Boss. 
It is, however, with the problem of the sun's distance, the third of the 
four great investigations carried on at the Cape, that Gill's name is so 
commonly and so intimately associated. 
We have already referred to his Mauritius and Ascension expeditions, 
and to the success that crowned his efforts on both occasions. It was to be 
expected that with his appointment to the Cape, and with the increased 
facilities which it would afford for astronomical enterprise, there would be 
a strengthening of his desire to determine once again the sun's distance. 
It seems to be a psychological law that no astronomer is ever wholly 
satisfied with his results, even when they are most dependable. His 
desire ever is to improve upon them. To the aid of this common 
inclination came the definite traditions of the Cape Observatory. 
One of the objects of Lacaille's visit to the Cape in 1751-1753 was to 
determine the sun's distance by declination differences of Mars. His. 
determination was accepted by astronomers for the greater part of a 
century, indeed remained unchallenged till Maclear in 1850, and again in 
1^62, improved on his value. 
When, forty years ago, Gill took up the problem, the uncertainty with 
regard to the sun's distance was no less than 10,000,000 miles. His first 
determination, made at Mauritius, reduced the range of uncertainty to 
within 1,000,000 miles. His Ascension results brought this margin down 
to 300,000 miles. 
Anxious to reduce even this amplitude of error to the narrowest limits 
possible, in 1883 he organized a concerted and highly complex scheme for 
improving the value of the solar parallax already obtained. Astronomers, 
generally, had by this time accepted his differential method as that which 
held the highest promise of accurate results. 
In 1888 an opposition of Iris, and in 1889 oppositions of Victoria and 
Sappho, furnished him with exceptional opportunities for such a deter- 
mination. His general plan was to cast behind these minor planets a 
network of stars, whose positions would be accurately determined by 
triangulating them together. Against this network the faint stellar discs 
of the minor planets chosen would be seen to pass, and the amount of 
