;iK DAVID GILL. 



197 



follow up the researches which had been broug-ht to so successful 

 a conclusion in Ascension, Gill desired that the extra meridian 

 appliances should be stipplemented, and to this end obtained by 

 purchase from Lord Lindsay the heliometer that he had used in 

 Mauritius and Ascension. He devoted his personal attention to 

 the working of this instrument, primarily with the object of deter- 

 mining with all available precision the annual parallaxes of a 

 series of stars, selected either on account of their brightness or on 

 account of their large proper motions, as likely to yield measurable 

 results. 



The researches were afterwards continued with a new and 

 more powerful heliometer furnished by the Lords Commissioners 

 of the Admiralty, in response to Gill's urgent representations. In 

 all the distances of some 22 stars were determined with one or 

 other or both of these instruments, the observations with the old 

 heliometer being shared by Gill with Elkin, who had volu'itarily 

 offered to accompany Gill to the Cape for the pur[)0se, and those 

 with the new instrument with Finlay and De Sitter. 



Gill has himself pointed out that in preparing the programme 

 of his second parallax-campaign he was guided not so much by 

 the desire to select such stars as would yield considerable paral- 

 laxes, but rather by the wish to make a contribution to cf smical 

 astronomy, which would throw light on the general distribution 

 of stars in space. A volume in which he summarises the results' 

 of his researches* contains an excellent discussion of the material 

 available at the time of its publication as bearing on this important 

 question. 



To elucidate the problems of cosmogony it w^as, however, 

 essentially desirable that more rapid, even if less accurate methods, 

 should be used. Gill w^as perhaps one of the first to recognise the 

 advantages of celestial photography as a means of securing data 

 on a wholesale scale rather than by the slow and laborious 

 methods of visual observations. 



The year 1882 was a memorable one in the annals of astron- 

 omy. The appearance of the great comet of that year was the 

 occasion for numerous attempts to secure photographs of this 

 remarkable object. For the most-part these attempts were of little 

 scientific value, as the cameras employed were not provided with 

 the means of following the comet through its diurnal movement. 

 However they showed effectively that the comet could be photo- 

 graphed. 



There was at the time no photographic equipment at the Cape 

 Observatory, but Gill secured the assistance of a local photo- 

 grapher, and with the aid of his camera strapped to the counter- 

 poiise of an equatorial telescope, so as to allow of motion to com- 

 pensate for the earth's rotation during exposure, excellent pictures 

 of the comet were secured. But what struck Gill more forcibly 



* Capf Antiah, vol. viii, part ii. 



