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PROCEEDINGS OF THE AMERICAN ACADEMY 



TABLE V. 



The brightest stars in the nebula are compared in Table V. The 

 first and second columns give the Bond number and magnitude. The 

 columns headed A, B, p], and F give the magnitudes derived from 

 those photographs respectively by the method of sequences. The re- 

 sults derived from E and F by the method of Argelander are given in 

 the columns headed E' and F'. The mean values of E and E', and of 

 F and F', are given in Table IV. 



One of the most important applications of the determination of pho- 

 to"-raphic magnitudes is to the measurement of the colors of the stars. 

 The rays affecting the photographic plate have in general a less wave- 

 leno-lh than those to which the eye is most sensitive. It therefore 

 follows that a reddish star, that is, one in which tlie rays of great 

 wave-length predominate, will appear relatively too faint in the photo- 

 graph. The residuals in the last columns of Table IV. will then be 

 positive. A bluish star is similarly indicated by a large negative resid- 

 ual. These residuals form a convenient measure of the color of the 

 stars. In most stars the difference in color is due to slight differences 

 in tlie relative intensities of the blue and red rays. Until the law 

 defining the relation of the intensity to the wave-length is known, a 

 single number serves to describe the principal cause of the color. Of 

 course in the case of stars in which a large part of the light is concen- 

 trated in bands or lines, the residuals will not be directly comparable 

 with those of other stars. Even here, however, this test may be ad- 

 vantageously employed to compare stars of the same class, as, for in- 

 stance, those of the third type of Secchi. 



