352 



SCIENCE. 



[N. S. Vol. VIII. No. 194. 



asked whether it would ever be possible to 

 photograph as faint a celestial bod\^ as 

 could be seen with a powerful telescope. 

 Here was the answer. It was not only 

 possible to photograph some of the faintest 

 objects seen in the telescope, but it was 

 possible also to photograph some others 

 which could never be seen in the sky. In 

 one of his reports Admiral Mouchez called 

 attention to the fact that the only way they 

 could see the satellite of Neptune at the 

 Paris Observatory was on the photographs 

 made by the Henry Brothers, for they had 

 no telescope sufficiently powerful to show it 

 visually. 



The Henrys applied themselves assid- 

 uously to celestial photography, with the 

 most remarkable success. They led the 

 world in this work. "While they were at 

 the height of their activity astronomers 

 elsewhere were but beginning to awaken to 

 the great importance of the subject. And 

 yet there seems to have been essentially no 

 public recognition of the work of these two 

 men, to whom astronomy owes so much. 

 In personal appearance and temperament 

 they are so extremely dissimilar that one 

 would scarcely take them to be brothers. 

 Up to the time they began their photo- 

 graphic work they had between them dis- 

 covered fourteen asteroids, by the slow and 

 tedious visual process of picking them out 

 by their motion from the countless thou- 

 sands of small stars. If one examines a 

 list of the asteroids he will be struck with 

 the manner in which these fourteen small 

 planets are recorded. According to this 

 list the first one of these was discovered in 

 1872 by Prosper Henry, the next one by 

 Paul Henry, and so on alternately through- 

 out the entire fourteen until 1879, when the 

 last one found was attributed to Paul 

 Henry. It is a curious fact, and one which 

 will be readily understood by all who are 

 acquainted with the unselfish affection ex- 

 isting between these two brothers, that the 



credit for the discovery of these fourteen 

 minor planets is ascribed alternately to 

 Prosper and Paul Henry. It is likely that we 

 shall never know which brother discovered 

 any one of these planets. 



Singularly enough, the photographic 

 plate not only did away with the necessity 

 of making these charts by eye and hand to 

 facilitate the discovery of asteroids, but it 

 also did away with the necessity of the 

 charts themselves for that purpose, for the 

 little planet, which is moving among the 

 stars, now registers its own discovery by 

 leaving a short trail — its path during the 

 exposure — on the photographic plate. The 

 first of these photographic discoveries of 

 asteroids was made by Dr. Max "Wolf in 

 1892. They are now found wholesale in 

 this manner by photography. 



It was the success of the Henry Brothers' 

 work that led to the International Astro- 

 Photographic Congress which met at Paris 

 in 1886. It was their work that caused 

 this Congress to meet at Paris. The Con- 

 gress adopted the Henry Brothers' lens as a 

 model for the instruments to be used, and 

 the work of this great undertaking was 

 based on that of the Henry Brothers. It 

 was stated once by Admiral Mouchez that 

 every object glass at the Paris Observatory 

 had either been made by the Henry 

 Brothers or refigured by them. 



Perhaps, as Dr. Dick himself thought in 

 the early days of photography, the most 

 unpromising subject for the photographic 

 plate to deal with was the nebulse. Most 

 of these objects appeared so feeble in their 

 light that but little encouragement in that 

 direction was offered the celestial photog- 

 rapher. 



One of the brightest and most promising 

 of the nebulse is that in the sword of 

 Orion, and this was naturally one of the 

 first of these objects to receive photographic 

 attention. In September, 1880, Dr. Henry 

 Draper began the photography of the nebu- 



