1887.] on the Applications of Photography in Astronomy. 159 



object-glass of short focus as the instrument best suited for the 

 purpose, and he states that, by mounting such a lens and camera on 

 an equatorial stand provided with clockwork, he has photographed 

 such groups of stars as the Pleiades, the chief difficulty being not to 

 fix the images of the stars, but to distinguish them from the specks 

 which are found on the plates or rather in the collodion. 



In 1864 Rutherford of New York completed a telescope of llrj 

 inches aperture and 14 feet focal length, specially constructed for 

 celestial photography, and obtained fine photographs of stars to the 

 9th order of magnitude. His remarks, although quoted in Mr. 

 Common's lecture last year, have such importance on the present 

 subject that I venture to repeat them. 



" The power to obain imagest of the 9th magnitude stars with so 

 moderate an aperture promises to develop and increase the application 

 of photography to the mapping of the sidereal heavens and in some 

 manner to realise the hopes which have so long been deferred and 

 disappointed. 



" It would not be difficult to arrange a camera-box, capable of 

 exposing a surface sufficient to obtain a map of two degrees square, 

 and with instruments of large aperture we may hope to reach much 

 smaller stars than I have yet taken. There is also every probability 

 that the chemistry of photography will be very much improved and 

 more sensitive methods devised." 



Mr. Common well remarks that in the light of recent work these 

 words are almost prophetic. 



But Rutherford did not stoj) here. In the eyes of an astronomer 

 a picture of stars is of comparatively little importance unless it is 

 capable of accurate measurement. Eecognising this important feature 

 of the case, Rutherford devised a suitable apparatus, which he ap- 

 plied to the measurement of two of his photographs of the Pleiades. 

 These measures having been put into the hands of Dr. Gould, that 

 astronomer compared them with those of the same group of stars 

 made by Bessel with his celebrated heliometer, and found a satis- 

 factory accordance.* 



Encouraged by these results, Dr. Gould, when he went to the 

 Argentine Republic in 1870 to found the Cordoba Observatory, which 

 has since been rendered so famous by his labours, took Rutherford's 

 telescope with him. Unfortunately one of the lenses was broken in 

 transj)ort, and such delay was incurred in replacing it, that the pro- 

 posed work could not be begun till 1875. But thanks to the clear 

 skies of Cordoba, and the marvellous activity of the observatory under 

 Dr. Gould's direction, 1350 photographs were obtained in course of a 

 few years, containing rej^resentations of all the principal star-clusters 

 of the southern hemisphere, besides a special series of plates taken 

 for the j)urpose of determining the parallax (or distance) of several 

 of the more remarkable stars in the southern hemis^Dhere. 



* AstroD. Nach. No. 162, vol. xlviii. Dec. 1866. 



