ADVISORY COMMITTEE ON ASTRONOMY 



1^55 



directors of these institutions, the use of telescopes of 40, 36, 26. 

 and 15 inches has been secured for a single investigation. The two 

 largest telescopes in the world are included in this work. The 

 standards for measurement have been selected from the vicinity of 

 known variable stars of long period. Charts of a large number of 

 these regions have been constructed b}- Father Hagen, on which all 

 the stars of the thirteenth magnitude and brighter have been entered. 

 Thirty-six of these regions have been selected, approximately at 

 equal intervals, but all north of declination — 30°. In each of these 

 regions five stars of the twelfth, fourteenth, fifteenth, and sixteenth 

 magnitudes are selected as standards. The twelfth-magnitude stars 

 have been measured with the twelve-inch telescope mentioned above. 

 Good progress has been made in measuring the fainter stars with the 

 larger telescopes. It is hoped that in two or three years we shall 

 thus be able to furnish standards of magnitude for the faintest stars 

 visible in the largest telescopes upon a uniform photometric scale. 



It will be seen from the above statement that the photometry of 

 the brighter .stars is better provided for than are some other depart- 

 ments of astronomy. For the faint stars a beginning only has been 

 made, and we have no means of judging whether the methods we 

 are using are good. The use of one or more telescopes of at least 

 two feet aperture is much to be desired. Other methods should also 

 be employed to check systematic errors. 



Various attempts have been made to determine the photographic 

 brightness of the stars. The first method, that of measuring the 

 diameter of the images, proposed by Bond in 1857, has perhaps been 

 more generally used than an\' other. Methods have been tried de- 

 pending on variation in aperture and exposure and on the use of 

 polarized light. If this problem can be solved, it ma)^ supersede 

 visual observations of magnitude. The fact that red. stars photo- 

 graph faint is rather an advantage, since a comparison with vi.sual 

 magnitudes gives also a definite measure of the color of the star. The 

 only complete solution of the problem of determining the colors of 

 the stars or other sources of light is by measuring the intensity of 

 different portions of the spectrum. This ma}' be done much more 

 easily by photography than visually. One of the principal objects 

 of photometry is to determine the distribution of the stars. Again, 

 photography has a great advantage over visual observations, espe- 

 cially in the case of faint stars. If possible, the stars should be 

 classified according to their physical condition as shown by their 

 spectra. In nearly all of this work the ultimate comparison is the 



