successful pictures of the comet were obtained, and on them a large number of stars were also shown. They were, as I have said, not by any means the first pictures of stars obtained by photography, but they represented in facility and in success so great an advance upon what had been formerly obtained that they attracted considerable attention. They were sent to Europe and stimulated various workers to further experiments. [Sidenote: The brothers Henry begin work.] [Sidenote: Conference of 1887.] The late Dr. Common in England, an amateur astronomer, began that magnificent pioneer work in astronomical photography which soon brought him the Gold Medal of the Royal Astronomical Society for his photographs of nebulæ. But the most important result for our purpose was produced in France. There had been started many years before by the French astronomer Chacornac a series of star maps round the Zodiac similar in intention to the Berlin maps which figured in the history of the discovery of Neptune. Chacornac died before his enterprise was very far advanced, and the work was taken up by two brothers, Paul and Prosper Henry, who followed Chacornac in adopting for the work the laborious method of eye observation of each individual star. They proceeded patiently with the work on these lines; but when they came to the region where the Zodiac is crossed by the Milky Way, and the number of stars in a given area increases enormously, they found the labour so great as to be practically prohibitive, and were in doubt how to deal with the difficulty. It was at this critical moment that these comet photographs, showing the stars so beautifully, suggested the alternative of mapping the stars photographically. They immediately set to work with a trial lens, and obtained such encouraging results that they proceeded themselves to make a larger lens of the same type; this again was satisfactory, and the idea naturally arose of extending to the whole heavens the scheme which they had hitherto intended only for the Zodiac, a mere belt of the heavens. But this rendered the enterprise too large for a single observatory. It became necessary to obtain the co-operation of other observatories, and with this end in view an International Conference was summoned to meet in Paris in 1887 to consider the whole project. There were delegates from, if not all nations, at any rate a considerable number:-- France 20 British Empire 8 Germany 6 Russia 3 Holland 3 U.S. America 3 Austria 2 Sweden 2 Denmark 2 Belgium 1 Italy 1 Spain 1 Switzerland 1 Portugal 1 Brazil 1 Argentine Republic 1 [Sidenote: Choice of instrument.] [Sidenote: Expense of "doublet."] [Sidenote: Advantages of reflector.] [Sidenote: Refractor chosen.]