September 5, 1902.] 



SCIENCE. 



367 



much valuable information. And since his 

 time Greenwich, Paris, Meudou, Mt. Hamil- 

 ton (Lick), Harvard, Yerkes and other 

 observatories have taken thousands of 

 plates. Many of my audience have seen 

 such pictures thrown on a screen by the aid 

 of a lantern, and thus have been able to 

 studj^ sun-spots, photosphere and faculaa in 

 a most instructive and accurate way. 



The earth's only visible satellite has 

 always stirred the interest of the astron- 

 omer. Schmidt, of Athens, Beer and 

 Madler, of Germany, and many others have 

 spent years of labor in making topograph- 

 ical drawings of the moon, and they pub- 

 lished very fine maps. Thirty years ago 

 Draper and Rutherfurd showed the world 

 what excellent photographs could be taken 

 with wet plates, and from that time many 

 of the great observatories have collected 

 hundreds of photographs of the moon on 

 the very sensitive dry plates of recent years. 

 We have now exquisite plates to study and 

 measure. Lately the French government 

 has published exact heliogravure copies and 

 enlargements of the lunar photographs 

 taken by Loewy and Puiseaux. 



In this connection it is proper to call 

 attention to the difference between an 

 object glass for seeing and one for photog- 

 raphy. The yellow rays affect the eyes most 

 readily and so the lenses must be ground to 

 bring those rays to a focus. But the blue 

 and the violet rays affect most the photo- 

 graphic film. So that with a telescope 

 arranged for seeing, the photographs ob- 

 tained, in most cases, are hazy and indis- 

 tinct. Rutherfurd, therefore, placed out- 

 side of his seeing object glass a lens of flint 

 glass, so arranged as to bring the blue rays 

 to a focus. It was with such a lens that he 

 obtained his fine plates of the sun, moon 

 and stars. 



To-day this same system of lenses is 

 mainly used, or a system involving the same 

 principle. Lately, however, there has been 



discovered at the Yerkes Observatory a new 

 method which gives great promise. When 

 the University of Chicago bought the forty- 

 inch object glass, they were unable to raise 

 the money to buy the needed extra lens, 

 which would enable them to photograph 

 well the moon and other heavenly bodies. 

 Fortunately, this was so, for it resulted in 

 experiments by Mr. Ritchey which demon- 

 strated the fine results to be obtained by a 

 screen. This screen of colored glass was 

 put in front of the sensitive plate, and al- 

 lowed only the yellow and red rays to pass 

 through the plate— it kept out the blue and 

 violet rays— and, therefore, only those rays 

 reached the sensitive plate which were ac- 

 curately focused by the object glass. The 

 result was some splendid photographs of 

 the moon and its details, as fine as anything 

 so far obtained. This discovery of the use 

 of a proper screen gives the promise of con- 

 verting any good seeing refracting tele- 

 scope into a fine photographic instrument 

 with very small expense. 



The reflecting mirror when properly 

 shaped brings to ' one focus all the rays 

 of light ; as well those rays which affect the 

 eyes best as those which produce the desired 

 result on the sensitive film. This fact has 

 brought into increasing use the reflectors of 

 large and small diameters. Modern methods 

 of producing and mounting silver-on-glass 

 mirrors have brought into considerable 

 prominence the reflector especially for pho- 

 tographic work. 



Gathering together all the photographs 

 made from the time of Rutherfurd (1874) 

 to the present, and later, will put into the 

 hands of the selenographer means of deter- 

 mining the changes on the moon. Changes 

 we most certainly expect. We are not 

 aware that there exists anything which does 

 not undergo change. But these changes 

 may be so small and so slow to us that it 

 may take years to discover them. 



The surroundings of the sun— that region 



