THE LIGHT OF THE STARS. 53 



the sky are comparable. A similar work on polar stars at upper and 

 lower culmination determined the photographic absorption of the at- 

 mosphere, 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 com- 

 plete for the portion of the sky extending from the equator to declina- 

 tion -j- 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 8,489 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 catalogue of photographic magnitudes is the ' Cape Photo- 

 graphic Durchmusterung,' the monumental work of Gill and Kapteyn. 

 454,875 stars south of declination — 19° are included in this work. 

 Unfortunately, the difficulty mentioned above, of reducing the magni- 

 tudes 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 

 reducing the measures of brightness to a standard scale can be over- 

 come, it will furnish the photographic magnitudes, as well as the posi- 

 tions, 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 photo- 

 graphic star image be measured by the amount of light it cuts off 

 from a thermo-pile, deserves careful study. It should give a great 

 increase in precision, and would eliminate that tool of many defects, 

 the human eye. No use seems to have been made so far of this method. 



The next question to be considered is, what use should be made of 

 these various measures of the light of the stars? The most obvious 

 application of them is to variable stars. While the greater portion of 

 the stars undergo no changes in light that are perceptible, several hun- 

 dred have been found whose light changes. A natural classification 

 seems to be that proposed by the writer in 1880. A few stars appear 

 suddenly, and are called new stars, or novas. They form class I. 

 Class II. consists of stars which vary by a large amount during periods 

 of several months. They are known as variable stars of long period. 

 Class III. contains stars whose variations are small and irregular. 

 Class IV. contains the variable stars of short period, and Class V. the 

 Algol variables, which are usually of full brightness but at regular 



