160 3Ir. David Gill [June 3, 



This fine series of pictures is now being submitted to measurement 

 by Dr. Goulcl, and the results are awaited with the greatest interest 

 by all astronomers. 



The first of Dr. Gould's plates were taken with the old wet 

 collodion process, but the work was afterwards greatly facilitated by 

 employment of the more sensitive modern dry plates. 



It was, in fact, the introduction of the gelatine dry plate process 

 in 1876, which really paved the way for the rapid development of 

 celestial photography. The convenience of the manipulation and tlie 

 great increase of sensitiveness of the i^lates at once placed a new 

 power in the hands of astronomers. Draper photographed the nebula 

 of Orion in 1880 ; and after trials, commencing in 1879, Common 

 succeeded in obtaining the exquisite photographs of that object which 

 have been exhibited more than once in this theatre. 



In 1882 appeared the splendid comet of that year. At the 

 Royal Observatory, CajDe of Good Hope, we were not at the time 

 engaged in photographic operations. Several photographers in 

 the Cape Colony found it possible to obtain impressions of the 

 comet, but they were unable to secure pictures of scientific value, 

 because they were unprovided with means to follow the diurnal 

 motion. I had no available camera belonging to the observatory, 

 and no experience in the development of modern dry plates. In 

 these circumstances, I applied to Mr. AUis, a skilful photographer 

 in my neighbourhood, who eagerly consented to co-oj^erate with me 

 in the work. I arranged means to attach his camera to the stand of 

 an equatorial telescope, and the telescope itself was employed to 

 follow the nucleus of the comet accurately during the whole time of 

 exposure by the aid of the driving clock and with small corrections 

 given by hand. The lens employed had an aperture of only 2 inches, 

 and a focal length of 11 inches; but the result was a series of 

 pictures, one of which, obtained after an exposure of two hours, is 

 now on the screen. 



The photograph shows a very satisfactory delineation of the tail 

 and envelope of the comet. 



Important and useful as these results were, there was another 

 feature of the pictures which seemed to me still more so. In 

 forwarding copies of these photographs to the Royal Astronomical 

 Society of London and to the Paris Academy of Sciences, I drew 

 particular attention to the large number of stars shown upon the 

 plate, and insisted upon the imj^ortance of the means thus offered to 

 photograph comparatively large areas of the sky and thus rapidly 

 make charts of the entire heavens. 



The one step wanting was now provided, and the new and more 

 sensitive dry plate rendered the former suggestions of de la Rue 

 and Rutherford now valuable and practicable. 



Formerly the old collodion wet plates required large instruments 

 (with small field) and long exposure to depict stars even to the 9th 

 magnitude, and astronomers trusted entirely to the accuracy of their 



