394 



SCIENCE. 



[N. S. Vol. VIII. No. 195. 



tronomy where photography has not taken 

 a prominent, if not a commanding position- 

 It is probable, however, that it will never 

 take the place of the micrometer in the ob- 

 servation of close double stars, and in this 

 direction the micrometer of Burnham will 

 perhaps never be displaced. The pho- 

 tography of the surface features of the 

 planets is in an almost hopeless condition 

 at present, yet much can be expected in 

 this direction when an increased sensitive- 

 ness of the plates has been secured. 



Photography has shown its value in the 

 determination of stellar parallax, and prob- 

 ably hereafter it will essentially take the 

 place of the micrometer in this direction. 



This is not the place to go into a discus- 

 sion of the relative values of the refractor 

 and reflector for photographic work. Where 

 accurate measurement is to be considered, 

 the refractor is doubtless better than the 

 reflector. If, however, the main object is a 

 great quantity of light, such as is required 

 for the photography of the nebulae, the 

 large aperture of the reflecting telescope of 

 short focus makes it, perhaps, the best form 

 of instrument (though it is very much 

 hampered by its small field). This has 

 been shown to be true by Common and 

 Roberts. Since in the reflector the light 

 does not pass through the glass, it is possi- 

 ble to use very large apertures without any 

 additional loss of light through absorption, 

 as would necessarily occur if it passed 

 through a large object glass. 



Mr. Ritchey, of the Yerkes Observatory, 

 is making a large glass speculum, five feet 

 in diameter and twenty-five feet focus, 

 which, when finished, will be one of the 

 most powerful instruments for photographic 

 and spectroscopic work yet made, and 

 which deserves a more extended notice 

 than my limited time will permit me to 

 give it here. With this instrument, and 

 Mr. Ritchey's skill in photographic work, 

 results of high importance will be obtained. 



Through the intelligent generosity o^ 

 Miss Catherine W. Bruce, of New York 

 Citj', astronomical photography has been 

 placed on a firmer basis than it ever was 

 before. Her gifts have been made to all 

 departments of astronomy, and it would 

 take considerable space to properly enumer- 

 ate them all. Perhaps the most important 

 of these are the ones that bear directly 

 upon astronomical photography. 



The first of these gifts was the great 

 24-inch photographic doublet by the 

 late Alvan Clark, presented to the 

 Harvard College Observatory, and which 

 is now doing such excellent work in 

 Peru ; the two 15-inch portrait lenses 

 for Dr. Max Wolf, of Germany, and a 10- 

 inch photographic doublet for the Yerkes 

 Observatory. These instruments are the 

 most powerful of their kind, and for cer- 

 tain classes of work are superior to any 

 other form of telescope. The results of 

 the splendid gifts of this lady must here- 

 after have the greatest influence upon the 

 higher development of astronomical pho- 

 tography. 



It is impossible within the limits of this 

 address to give more than a general, and at 

 best incomplete, sketch of the rise and prog- 

 ress of photography in the various lines of 

 astronomical research. To those who have 

 kept pace with these rapid strides in the 

 last twenty years this brief history will 

 seem imperfect, and perhaps of little inter- 

 est. Many applications of the photo- 

 graphic art and many valuable results have 

 necessarily been omitted. But few of the 

 names of those prominently identified with 

 this subject have been mentioned, and but 

 little of their work even alluded to. A 

 volume of no small dimensions would be 

 necessary to give a complete history of the 

 development of photography in the many 

 directions in which it has been applied to 

 astronomy. The time to do this has not yet 

 come. Progress has been so rapid and far- 



