162 Mr, David GUI [June 3, 



solid existing basis for founding any theory as to the constitution of 

 the stellar universe. To complete the Durchmusterung for the re- 

 maining portion of the heavens was therefore the most pressing need 

 of modern astronomy. I commenced the work in 1885 by the aid of 

 photography. I hope in two or three years, if I have the honour of 

 lecturing again in this theatre, I shall then be able to tell you that the 

 work in question is finished. 



I should here explain that mere pictures of the stars are of com- 

 paratively little value, or rather of about the same value to an astro- 

 nomer as a series of charts of parts of the world would be to a sailor if 

 there were no lines of latitude or longitude marked upon them. 



The every-day useful part of the Durchmusterung is the catalogue 

 giving the positions and magnitude of all the stars. That work is 

 rapidly advancing in the hands of my able and enthusiastic friend, 

 Professor Kapteyn of Groningen, who, with the aid of three 

 assistants, has undertaken to devote five or six years of his life to the 

 measurement of the Cape photographs and the computation of the 

 results. 



When this has been done, as I venture to think it will be within 

 five years, astronomers will be in possession of that preliminary 

 survey of the whole heavens which is necessary for the more refined 

 and elaborate researches which must follow as results of the Paris 

 Congress. 



But to return to the work that was meanwhile being done in Paris 

 by the brothers Paul and Prosj)er Henry. 



These astronomers had been engaged since 1871 in the construc- 

 tion of charts of the Ecliptic by the older processes of observation, 

 but when they reached that portion of the heavens where the Milky 

 Way crosses the Ecliptic, the number of stars became so overwhelming 

 that the task of charting seemed almost too great for human patience 

 and skill. But fortunately the time had come when dry plate photo- 

 graphy could be called in to aid, and this aid was in the hands of men 

 singularly competent to develop such an opportunity to the fullest 

 extent. The brothers Henry had long aspired to be not only dis- 

 tinguished practical astronomers, but, following the traditions of 

 Huyghens and the Herschels, they desired also to be the artists of 

 their own optical means. Bound together by strong brotherly affection 

 and common tastes, gifted alike with practical talents of a high order, 

 and with an energy and determination of character that permit no 

 obstacle to success, these men thus happily united have devoted the 

 spare hom's of their busy astronomical duties at the Paris Observatory, 

 first to the study of optics, and afterwards to the grinding and polish- 

 ing of lenses and specula, which have won for them a now world-wide 

 reputation as opticians of the highest rank. 



I had the pleasure, a few weeks ago, of visiting the modest work- 

 shop attached to their house at Montrouge, and I shall not soon 

 forget that visit, nor the many lessons moral as well as practical 

 which I learned. 



