ADVISORY COMMITTKE ON ASTRONOMY 157 



is therefore only necessary to determine once for all the nature of the 

 variation, and then occasionally determine the times of maximum 

 light to see if the period has undergone any change. The variations 

 can be determined so accurately by photometric measurements that 

 visual observations are of little value. The light curves of a large 

 part of them have already been determined at Harvard, and it is now 

 only necessary to measure these stars from time to time. Some of 

 them vary so rapidly that the periods of variation can be found with 

 great accuracy. An attempt is made at Harvard to measure each 

 star which changes more than half a magnitude an hour, on three 

 nights each year when it is increasing in light, and on an equal 

 number when it is diminishing. The same remarks apply with even 

 more force to stars of the Algol type. If all the light curves can be 

 determined once for all, occasional observations only will be needed 

 later. 



We may therefore conclude that appropriations are much needed 

 to secure the required observations of variable stars of long period, 

 but that sufl&cient provision is now made for the photometric study 

 of other variable stars. Spectroscopic observations of such stars, 

 which are greatly needed, are referred to in the section on spectros- 

 copy. 



Celestial Photography. 



The application of photography to astronomy by Bond in 1850, 

 and again in 1857, was an advance comparable in importance with 

 the invention of the telescope. Bond's experiments were made with 

 a visual telescope ill-adapted for the work. The production of the 

 first photographic refractor by Rutherfurd in 1864 enabled him to 

 obtain many remarkable photographs of the Sun, Moon, and stars. 

 His negatives were used by Gould in 1866, and later by Jacoby and 

 others in the precise measurement of such stellar groups as the 

 Pleiades. In 1880 Draper obtained the first photograph of the 

 Great Nebula in Orion. Up to this time the use of wet plates had 

 greatly hampered the work, but the introduction of the much more 

 sensitive dry plates led at once to important advances, especially at 

 the Cape, at Paris, and at Harvard. At the Cape of Good Hope, 

 Gill photographed the great comet of 1882 with an ordinary portrait 

 lens strapped to a telescope. The immense number of stars recorded 

 on the plate led to the publication in 1896 of the Cape Photographic 

 Durchmusteru7ig , which gives the approximate positions and magni- 



