STELLAR PnOTOGRAPHY. 



215 



226. Plate 248. A second measure gave 7.0 instead of 6.2. 

 The mean value 6.6 has been adopted. The original 

 residuals were 35 11. Sjiectrum uncertain; 227 inter- 

 feres. 



141. Spectrum probably Type I, but not separated from 143. 



213. Spectrum uncertain ; 20S interferes. 



217. Spectrum imcertain ; 227 interferes. 



447. Spectrum T)'pe I, but the K line is visible, and about 0.2 

 as intense ct the H line. 



235. 227 interferes. 



358. Plate 209. A second measure gave 7.0, but a more care- 

 ful estimate gave 7.5 instead uf the original measure 8.0. 

 The mean value 7.5 has been adopted. The original resid- 

 uals were 4 2 51. Spectrum uncertain ; 349 interferes. 



208. Residuals in o, 11 ; in />, 10. 



120. Spectrum Type I. Presence of K line doubtful on ac- 

 coimt of interference of 115. 



209. Spectrum uncertain ; 212 interferes. 



192. Spectrum Tyjie I, but too faint to decide whether the 



K line is present. 



280. Spectrum faint ; lines narrow, if present. 



219. Spectrum imcertain; 214 interferes. 



376. Residual in c, 10. 



101. Spectrum not seen; 103 aud 115 interfere. 



198. Spectrmn not seen ; 213 and 217 interfere. 



11. Spectrum faint. 



407. Residuals in a, 12 ; in 6, 14 ; in c, 15. 



109. Spectrvmi not seen ; 105 interferes. Residuals in a, 16; 



in 6, 17 ; in c, 18. 



143. Spectrum probably Type I, but not separated from 141. 



202. Spectrum not seen ; 212 interferes. 



103. Spectrum not seen ; 105 interferes. Residual in a, 12. 



204. Spectrum uot seen ; 213 and 217 interfere. 



The spectra of Nos. 169 and 245 were seen, but were too faint to indicate their 

 type. No trails were given by the stars 26, 11, and 5 Pleiadum, which have the 

 numbers 245, 169, and 107 in Wolf's Catalogue. 



An important inference may be drawn from the comparison of the spectra of the 

 stars of this group. It is extremely improbable that chance alone has brought 

 together so many bright stars in the same portion of the heavens. Most of them 

 probably have a common origin, and are much nearer to each other than to the 

 Solar System. A few, doubtless, have only an apparent connection with the group, 

 their real distance being much greater or less than that of the others. Ordinary 

 means fail to distinguish the individuals of these two classes. The similarity in 

 the chemical and physical conditions indicated by the apparent identity of most of 

 the spectra, is a strong confirmation of their common origin. The variation in the 

 spectra of such stars as Nos. 338 and 447 seems to indicate that these stars happen 

 to lie in the same direction from us as the others, but are not really connected 

 with them. In a study of the parallax of the Pleiades, it seems very desirable that 

 these stars also should be observed. 



To determine the probable error of a single determination of brightness, those 

 stars only should be included which are measured on all four plates. The original 

 uncorrected residuals are also used even in those cases where a second measure 

 showed that the first estimate was erroneous. The 140 residuals of the 35 stars 

 included in this list give an average deviation of ±0.119 magnitudes. Using the 

 corrected residuals, the average deviation would be reduced to ±0.106. The prob- 

 able error of a single estimate will be 0.119 X 0.976, or ±0.12, and the error of a 

 single star ±U.06. Of the residuals, 47 have the value 0; and of the positive resid- 

 uals, 23, 15, 8, and 3 have the values +0.1, +0.2, +0.3, and +0.4, respectively. Of 



