president's address SECTION A. 93 



astronomy was coming to the front ; but it is evident that he had 

 no adequate conception what it was going to do for exact records- 

 or for descriptive astronomy, or we should have had his great 

 powers devoted to its development. But who could dream in those 

 days that it would be possible now to say, as Professor Pritchard^" 

 has said, that in measuring distances of over 2,000 seconds of arc for 

 his photo-parallax experiments he had found the probable error of 

 the distance between two stars so measured to be only one-tenth of 

 a second of arc, and that the camera and spectroscope combined, in 

 Professor Yogel's hands, had separated a double star with a dis- 

 tance of only six-thousandths of a second of arc — a quantity so 

 small that our great telescope will have to be enlarged thirtyfold 

 before we can see it. And Professor Vogel's determination of 

 star motions in the line of sight has, in the opinion of competent 

 persons, shown that attempts to determine the motions of stai's in 

 the line of sight without tlie aid of photography was little better 

 than a waste of time. And Professor Keelei"*^', recently in charge 

 of the great Lick telescope, and therefore having full knowdedge of 

 the powers of the greatest telescope in the world, writes it has been 

 shown " that visual observation of the spectrum cannot in general 

 compete with photographic methods applied to the same as even 

 much smaller telescopes." Indeed no one can study the results^ 

 obtained by photography where it has been fully applied without 

 being impressed by the fact that the results are not only far in 

 excess of the amount possible by eye observation, but also of far 

 higher value, and that after a time photography will displace the 

 observer from all astronomical instruments and do much better 

 work than he could ever hope to do with his eyes. 



We have to-day passed in hurried review the application of 

 photography to the wants of the astronomer in delineating the 

 moon's surface in the study of her libration; to recording the sun's 

 disc, his spots, faculse, rice grains, photosphere, red jjrominences, 

 the corona in actual and in artificial eclipse; to the sun's motion 

 in space ; to the sun's rotation periods ; to recording that wonder- 

 ful spectrum with its thousands of lines ; to the record of double 

 stars ; star charting ; star magnitudes ; to their classification by 

 quality of light ; to recording their almost inconceivable numbers ; 

 to star drifting ; to star motions in the line of sight ; to double 

 stars so close and so remarkable that they can only be recorded by 

 this means ; to the record of all the visible stars in the sky for the 

 purpose of detecting changes of magnitude : to the record of the 

 spectrum of every star doAvn to the tenth magnitude; to finding 

 invisible stars and invisible lines in their spectra ; in recording the 

 forms and details of nebulse ; to their spectra, to show that the eye 

 does not see all details they present nor their extraordinary exten- 

 sion ; its application to rec 'I'ding the form and appearances of 

 comets ; to the record of the invisible rays in their tail ; to their 

 spectra; to the surface-marking of planets; to their spectra; to 



