380 ASTRONOMY 



We have as yet only considered the total light of a star, so far as 

 it affects the eye. But this light consists of rays of many different 

 wave-lengths. In red stars, one color predominates, in blue, another. 

 The true method is to compare the light of a given wave-length in 

 different stars, and then to determine the relative intensity of the 

 rays of different wave-lengths in different stars, or at least in stars 

 whose spectra are of different types. This is the only true method, 

 and fortunately spectrum photography permits it to be done. The 

 Draper Catalogue gives the class of spectrum of 10,351 stars, and 

 the relative brightness of the light whose wave-length is 430 is deter- 

 mined for each. In 1891, measures were published of the relative 

 light of rays of various wave-lengths, for a number of stars whose 

 spectra were of the first, second, and third types. 



A much simpler but less satisfactory method is to measure the 

 total light in a photographic image. As in the case of eye-photometry, 

 this method is open to the objection that rays of different colors are 

 combined. Blue stars will appear relatively brighter, and red stars 

 relatively fainter, in the photograph than to the eye. This, however, 

 is an advantage rather than an objection, since it appears to furnish 

 the best practical measure of the color of the stars. Relative photo- 

 graphic magnitudes can be obtained in a variety of ways, and the 

 real difficulty is to reduce them to an absolute scale of magnitudes. 

 But for this, photographic might supersede photometric magnitudes. 

 In other respects, photography possesses all the advantages for this 

 work that it has for other purposes, and many photometric problems 

 are within the reach of photography, which seem hopeless by visual 

 methods. In 1857, Professor George P. Bond, the father of stellar 

 photography, showed that the relative light of the stars could be 

 determined from the diameter of their photographic images. This is 

 the method that has been generally adopted elsewhere in determining 

 photographic magnitudes, although with results that are far from 

 satisfactory. It is singular that although this method originated at 

 Harvard, it is almost the only one not in use here, while a great 

 variety of other methods have been applied to many thousands of 

 stars, during the last eighteen years. Relative measures are obtained 

 very satisfactorily by applying the Herschel-Argelander method to 

 photographic images, and if these could be reduced to absolute mag- 

 nitudes, it would leave but little to be desired. In the attempt to de- 

 termine absolute magnitudes a variety of methods has been employed. 

 The simplest is to form a scale by photographing a series of images, 

 using different exposures. The image of any star may be compared 

 directly with such a scale. To avoid the uncertain correction due to 

 the time of exposure, different apertures may be used instead of dif- 

 ferent exposures. Another method is to attach a small prism to the 

 objective. The image of every bright star is then accompanied by 



