PHOTOGRAPHY IN TUt SERVICE OF ASTRONOMY. 47 1 



Warreu de La Rue, and later in America, witli a success contiuually in- 

 creasing-, by Mr. Eutherfurd and by Mr. B. A. Gould, charged with the 

 direction of the Cordoba Observatory under the tine sky of the Argen- 

 tine llepublic. Gould began his work in this line about 1875 and sue 

 ceeded in gathering-, in a few years, a collection of more than a thousand 

 stellar photographs of the highest interest. After having- tested, him- 

 self, the slowness of the wet collodion process, he was able at a later 

 date to utilize bromide of silver gelatine dry ])latevS, the invention of 

 which marked a new phase in celestial photograi)hy.* It is necessary, 

 finally, to mention here the attempts in stellar i>hotography of Henry 

 Draper, of Ainslie Common, and Isaac Roberts, who have studied the 

 resi)ective advantages of refractors and retiectors with silvered mirrors; 

 of Pickering, who has constructed for the H:u'vard College observatory 

 at Cambridge, United States of America, a photographic equatorial 

 specially designed for the ra[)id execution of celestial charts on a mod- 

 erate scale; of David Gill, the eminent director of the observatory of 

 the Cape of Good Hope, who commenced in 1885 a photographic revis- 

 ion of the southern sky, comprising the stars to the ninth or tenth 

 magnitudes, similar to the catalogue pre[>ared by Argelander for the 

 northern sky. 



At the Paris Observatory like labors have been also pursued for some 

 years with most marked success. Paul and Prosper Henry had under- 

 taken, in 1871, to continue the '•ecliptic chart" commenced by Chacor- 

 nac who had been able to execute it only in part. This chart, extremely 

 useful in searching- for small planets, which was to contain all stars to 

 the thirteenth and fourteenth magnitudes, extended along the ecliptic 

 in a zone live degrees in breadth. Now at a certain point the Henry 

 brothers found themselves arrested in this work by the manifest im- 

 possibility of constructing, by old processes, the sections of the charts 

 where the swarming of the stars announces the approach of the Milky 

 Way. It w^as then that they decided to resort to photography. They 

 were, says Admiral Mouchez, admirably prepared to con<iuer these dif- 

 ficulties. " Following the traditions to-day too much forsaken, of great 

 astronomers of former times who employed their own hands in the con- 

 struction of their instrunu^nts, they devoted for a long time, in their 

 modest work-shoj) of Montrouge, all the moments of liberty which weie 

 left them from their very active service at the Paris observatory, in the 

 study of the grinding and i)olishing of opti(;al glass. An extensive ac- 

 quaintance with the questions for solution, the harmony of fitness some- 

 what different and very happily associated in two brothers, an ener- 

 getic will and a i)ersevering labor, which no distraction ever chanced 

 to trouble, could not fail in assuring for them a well-merited success. 

 They became, in a few years, the most skillful artists of France, and 

 their fame was no less great among foreigners." After having con- 



* Eayet : ' ' Notes on tbe history of astrouomical photography." (Bulletin astronomique 

 t. IV, p. 318.) 



