1887.] on the Applications of Photography in Astronomy. 161 



driving clocks, which could not follow a star with perfect accuracy 

 during a long exposure. Now the modern rapid dry plates in con- 

 junction with the large fields of the j)hotographic objective overcame 

 the first of these difficulties, and the plan of employing a guiding 

 telescope overcame the second. 



The use of a guiding telescope was not even a new device, for it 

 had been employed long before by Hartnup and others, who, in their 

 early attempts to photograph the moon, kept the image of a lunar 

 spot by hand upon the cross wires of the finder of the telescope during 

 the long exposures then necessary. 



There was thus nothing really new either in my suggestion or in 

 the modus operandi, only the result was a fortunate one, for Mr. 

 Common says that " these photographs came to him as a revelation of 

 the power of photography for the purpose of star-charting," * and 

 Admiral Mouchez tells me that these Cape photographs and my sug- 

 gestions first directed his attention and that of the brothers Henry 

 to the application of photography to the work of star-charting, which 

 had for many years been carried on at Paris by the older methods of 

 astronomy. 



Common was amongst the first to take up the work in England, 

 and here on the screen is one of his photographs with a 4-inch 

 lens, executed in December 1883. But being engaged in other 

 researches, Common made no attempt to commence a systematic 

 survey of the heavens. 



Isaac Roberts, of Liverpool, was also early at work in the same 

 field, and after preliminary experiments he acquired a powerful 

 telescope, with which he began a systematic survey of the northern 

 heavens. 



It required some time to find the necessary means and apparatus 

 to begin the realisation of my ideas at the Cape, but at last the work 

 was started in the beginning of 1885 on the following definite plan, 

 viz. to complete the cartography of the heavens from 20° south of 

 the Equator to the South Pole, and so as certainly to include all 

 stars to the 9th magnitude. 



The reasons for the adoption of this plan were the following : — 



The celebrated astronomer Argelander charted the heavens on 

 this scale from the North Pole to the Equator, and the work has 

 recently been extended to 20^ south of the Equator by Schonfeld, the 

 pupil and successor of Argelander. 



Argelander's Diirchmiisterung, as it is called, has furnished, ever 

 since the date of its publication, the nomenclature of all the fainter 

 stars employed in the daily operations of astronomy ; it has furnished 

 the working catalogues which are essential for the more exact deter- 

 mination of the places of all these stars ; it has given us the first 

 accurate data for determining the distribution of the stars according 

 to magnitude and apparent position in the heavens, and is the first 



* ' Proc. Koyal Institution,' vol. xii. part ill. p. 734. 

 Vol. xii. (No. 81.) m 



