514 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



of a thorough overhaul. He at once set to work upon it with his 

 usual energy, and so transformed it that for differential work it left 

 little to be desired. The Airy transit circle performed useful service 

 until 1901, when it was replaced by the new reversible transit circle. 

 It is still used at times for special researches. The 7-inch equatorial 

 was likewise submitted to a thorough overhaul. 



The only immediate addition to the equipment was the 4-inch 

 heliometer, which was secured by Gill by private purchase. With 

 this provision he was content to spend the first few years of his 

 directorship, until he should be in a stronger position to press his 

 claims on the treasury. The principal additions made during the 

 subsequent years that he spent at the Cape were the 6-inch Dallmeyer 

 lens used for the photographic Durchmusterung, acquired in 1884; 

 the 7-inch heliometer in 1887, the astrographic refractor erected in 

 1890, the Victoria telescope (a 24-inch photographic refractor with 

 guiding telescope and spectroscopic equipment) in 1898, and the 

 reversible transit circle in 1901. He was thus for his first researches 

 limited to instruments of very moderate size and cost, and the suc- 

 cess wdth which he afterwards obtained an adequate provision for 

 the observatory was due both to the confidence inspired by his bril- 

 liant early work and to his pertinacity in pressing the needs of 

 astronomy. 



If from his many and varied services to astronomy we were asked 

 to pick out the one in which he arrived at the most striking and 

 complete success, there is little doubt that the answer would be his 

 determination of the solar parallax. At the time when Gill, by 

 accepting the charge of the Dun Echt Observatory, definitely em- 

 barked on an astronomical career a celestial event of the first magni- 

 tude was approaching — the transit of Venus of 1874. Great expecta- 

 tions were entertained that this w^ould afford an improved determina- 

 tion of the solar parallax, a fundamental constant which was at that 

 time involved in unsatisfactory uncertainty. Preparations were 

 made by the leading observatories and astronomical societies on an 

 unprecedentedly lavish scale, and expeditions were dispatched to 

 different parts of the world. Lord Lindsay was cooperating in the 

 work, and the Dun Echt expedition took up a station at Mauritius. 

 Gill had already formed the opinion (which he afterwards conspicu- 

 ously advocated) that there were other and better methods of find- 

 ing the sun's parallax involving far less expense. He believed that 

 the observations of the transit were of such a nature that the results 

 would be inaccurate and capable of more than one interpretation, 

 for too much depended on the arbitrary judgment of those who had 

 to discuss the observations. He determined, therefore, to use the 

 opportunity of the expedition to make trial of another method, 

 namely, morning and evening observations of the minor planet 



