THE LIGHT OF THE STARS 381 



a second image a few minutes of arc distant from it, and fainter by a 

 constant amount, as five magnitudes. Trails may be measured more 

 accurately than circular images, and trails of stars near the pole have 

 varying velocities, which may then be compared with one another by 

 means of a scale. Again, images out of focus may be compared with 

 great accuracy and rapidity by means of a photographic wedge. 

 These comparisons promise to furnish excellent magnitudes, if they 

 can only be reduced to the photometric scale. A catalogue giving the 

 photographic magnitudes of 1131 stars within two degrees of the 

 equator, and determined from their trails, was published in 1889. 

 Great care was taken to eliminate errors due to right ascension, so 

 that standards in remote portions of the sky are comparable. A sim- 

 ilar work on polar stars at upper and lower culmination determined 

 the photographic absorption of the atmosphere, which is nearly 

 twice as great as the visual absorption. A catalogue of forty thousand 

 stars of the tenth magnitude, one in each square degree, has been 

 undertaken, and the measures are nearly complete for the portion of 

 the sky extending from the equator to declination -f 30. These stars 

 are compared, by means of a scale, with the prismatic companions of 

 adjacent bright stars. Two measures have been made of images out 

 of focus of 8489 stars, including all of those north of declination 

 20, and brighter than the seventh magnitude. This work is being 

 continued to the south pole. The most important completed cata- 

 logue of photographic magnitudes is the Cape Photographic Durch- 

 musterung, the monumental work of Gill and Kapteyn. 454,875 

 stars south of declination 19 are included in this work. Unfortun- 

 ately, the difficulty mentioned above, of reducing the magnitudes 

 to an absolute system, has not been wholly overcome, but the 

 work is published in a form which will permit this to be done later, if 

 a method of reduction can be discovered. The extension of this great 

 work to the north pole is one of the greatest needs of astronomy at 

 the present time. 



The map and catalogue of the Astrophotographic Congress, the 

 most extensive research ever undertaken by astronomers, will not be 

 discussed here, as it will doubtless be described by others better able 

 than I to explain its merits. If completed, and if the difficulty of re- 

 ducing the measures of brightness to a standard scale can be overcome, 

 it will furnish the photographic magnitudes, as well as the positions, 

 of two million stars. Time does not permit the consideration here of 

 certain other investigations of photographic magnitudes, such as those 

 made at Groningen. They generally relate to a comparatively small 

 number of stars. . 



The suggestion that the intensity of a photographic star-image be 

 measured by the amount of heat it cuts off from a thermopile, deserves 

 careful study. It should give a great increase in precision, and 



