8 SCIENCE PROGRESS 



the unfocussed blue rays, leaving only those for which the 

 telescope has been properly focussed. When a suitable plate 

 is then put behind the screen, pictures of the moon and stars 

 can be and have been obtained quite as good as those obtained 

 with a telescope specially made for photography. But in 1882 

 this had not been realised and the Brothers Henry saw no way 

 of using the new and promising photographic method but to 

 make a new lens specially adapted for it. This they set about 

 with great skill and determination. After a few trials on small 

 lenses they at last succeeded in producing a photographic lens 

 of 13 inches aperture, a veritable triumph of optical workmanship 

 at that time. They were of course amateurs at the work. 

 Admiral Mouchez, the Director of the Paris Observatory, gave 

 them every encouragement and put at their disposal such 

 resources as he had available ; but their workshop was after 

 all a mere shed. I have often heard Dr. Common speak with 

 amusement of his visit to the workshop which had turned out 

 to the admiration of the world the first successful photographic 

 refractor — the modest building and the humble appliances were 

 so surprising. We are reminded of the simple apparatus with 

 which great experimenters like Faraday have often achieved the 

 most remarkable results. 



It was the work of the lens thus produced by the Henrys 

 that led directly to the inception of the project we are 

 considering. The specimen maps of small regions of the sky 

 which they soon obtained suggested the possibility of producing 

 such maps for the whole sky. The work contemplated was no 

 child's play. At least 10,000 maps would be required to cover 

 the whole sky ; and a labour of this magnitude was beyond 

 the resources of a single observatory. Correspondence between 

 Sir David Gill — under whose direction the comet photographs 

 had been taken — and Admiral Mouchez, who had encouraged 

 the work of the Henrys, led ultimately to the assembling of 

 a great international Conference at Paris in 1887. It was a 

 remarkable meeting, the first of its kind in the history of 

 astronomy; and it has shown the way for subsequent gatherings 

 which have already made their mark upon that history. Con- 

 ferences of a similar kind have since been held in 1889, 1891, 

 1896, 1900; and after a long interval in 1909. On all these 

 occasions the French have acted as hosts and have discharged 

 these duties with a cordiality and hospitality that has never 



