52 POPULAR SCIENCE MONTHLY. 



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. Eelative 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 seems hopeless by visual meth- 

 ods. In 1857, Professor George P. Bond, the father of stellar photog- 

 raphy, 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 al- 

 most the only one not in use here, while a great variety of other meth- 

 ods have been applied to many thousands of stars, during the last eigh- 

 teen years. Eelative measures are obtained very satisfactorily by ap- 

 plying the Herschel-Argelander method to photographic images, and 

 if these could be reduced to absolute magnitudes, it would leave but 

 little to be desired. In the attempt to determine 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 times of exposure, differ- 

 ent apertures may be used instead of different exposures. Another 

 method is to attach a small prism to the objective. The image of every 

 bright star is then accompanied by a second image a few minutes of 

 arc distant from it, and fainter by a constant amount, as five magni- 

 tudes. 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, im- 

 ages 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 photo- 

 metric scale. A catalogue giving the photographic magnitudes of 

 1,131 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 



