SIR DAVID GILL — EDDINGTON. 519 



of eliminating this peculiarly difficult source of error by screens and 

 other methods, and it was a source of great satisfaction to him that 

 the traveling-wire micrometer seems to have successfully accom- 

 plished this object. 



Keference has already been made to Gill's early photographs of 

 the moon. These were, of course, not by any means the first lunar 

 photographs, but in 1882 Gill made a notable advance in celestial 

 photography by successfully photographing the great comet of that 

 year. Several pictures of this comet had already been obtained, 

 with fixed camera, and the knowledge thus obtained that the light 

 was sufficiently intense encouraged Gill to attempt to obtain images 

 of greater scientific value by guiding the camera in the modern way. 

 He was assisted by Mr. Allis, a local photographer, from whom he 

 borrowed a doublet of 2^ inches aperture and 11 inches focal length. 

 He mounted this doublet on the 6-inch equatorial, which he used as 

 guiding telescope. Excellent representations of the comet were ob- 

 tained with exposures of from 30 minutes upwards; but, a fact of 

 still greater importance, it was found that, notwithstanding the in- 

 significant size of the apparatus, a great many stars were shown 

 whose images were well defined over a large field. This suggested the 

 practicability of using similar but more powerful instruments for 

 mapping the sky and for other astronomical purposes to which 

 photography is now applied. 



We now know how this result has revolutionized the methods of 

 observational work. Gill led the way in turning the new possibilities 

 to a practical account. The immediate outcome was the Cape Photo- 

 graphic Durchmusterung, started in 1885. The survey covers the 

 region of the sky from the South Pole to Dec. - 18°, and is complete 

 so far as photographic magnitude 9"^-2 (on the C.P.D. scale). A 

 rapid rectilinear Dallmeyer lens of 6 inches aperture and 51 inches 

 focal length was used for the photography. The work was completed 

 in 1890. Very soon after the start Prof. Kapteyn's offer was received 

 to devote himself for some years to the arduous labor of the meas- 

 urement and reduction of the plates, a work for which the Cape Ob- 

 servatory was unable to provide. This is a further instance of GilFs 

 success in attracting for his helpers the men best capable of carrying 

 out the work desired. The association of Gill and Kapteyn, which 

 began now, has proved a most powerful influence in the advance of 

 stellar investigation, and, to quote Gill's own words, "probably the 

 most valuable result of the C.P.D. to science is the fact that it first 

 directed Kapteyn's mind to the study of the problems of cosmical 

 astronomy and thus led him to the brilliant researches and dis- 

 coveries with which his name is now and ever will be associated." 



We can only mention briefly the other photographic work with 

 which Gill was associated. When the history of the inception of the 



