1888.] Silver on Glass Mirrors of different Focal Lengths. 181 



Attention may here be drawn to the precision of the results 

 obtained by measures so independent of each other. As an accidental 

 result of these recent measures of the photographic magnitude of the 

 stars, it may be mentioned that in May, 1886, the photographic 

 magnitude of Star 22 in the Pleiades was 0*35 magnitude less 

 than the photometric as obtained from very many measures, and I 

 attributed this difference to the probable actinic peculiarity of the 

 star in question, but inasmuch as no such perceptible difference exists 

 in the more recent measures of the photographic and photometric 

 magnitudes, resulting as they do from so many independent deter- 

 minations, the question of the variability of this star is suggested as 

 very probable. Pleione also in the measures of 1886 exhibits a 

 difference between the photometric and the photographic magnitude. 

 Inasmuch as the same difference in the measures has been again 

 exhibited in the recent measures, it seems reasonable to explain the 

 fact by the peculiar actinic action in the light of this star. 



As a further example of the power and applicability of this 

 definite method in reference to faint stars not suitable for determina- 

 tion by the wedge photometer, I may add here the following 

 comparison of the resulting measures made by the photographic 

 method, set side by side with the magnitude as estimated by Wolf 

 (' Description da Groupe des Pleiades,' Paris, 1874). 



Star's 

 designation 

 No. in Wolf. 



Measured 

 diameter. 



Computed 

 (photographic) 

 magnitude. 



Estimated 

 magnitude. 

 Wolf. 



Difference 

 C - O 

 in mag. 



196 





9'72 



10 



-0-28 



314 



9-98 



9 61 



10 



-0-39 



239 



6-04 



11-51 



11 



+ 0-51 



211 



5 85 



11-60 



11 



+ 0-60 



318 



8-15 



10-45 



11 



-0-55 



319 



8'40 



10-34 



11 



-0-66 



325 



5-67 



11-70 



12 



-0-30 



330 



6-14 



11 -45 



12 



-0-55 



331 



5 35 



11 -87 



12 



-0-13 



320 



4-47 



12-35 



13 



-0-65 



321 



3-98 



12-63 



13 



-037 



332 



4-23 



12-49 



13 



-0-51 



302 



3 50 



12-91 



14 



-1-09 



324 



doubtful 





14 





It has been more than once proposed to estimate or to measure the 

 photographic magnitudes of stars, by means of the breadth and 

 character of their traces on the photographic plates. This method 

 would involve an unnecessary consumption of time in procuring 



