328 Rutherfurd Photographic Measures. 



that we have the following average probable errors : 

 for the declinations : 



lA ["-°4] 2 + i [".o6]° = ± o".o 5 



for the right ascensions : 



sec 8 y i [".04] 2 + i [".06] 2 = ± o".o6. 



These are the average probable errors of the co-ordinates given in 

 the final catalogue of Section VIII., so far as they can be derived 

 from the internal agreement of the measures upon the various plates. 

 They show that extremely little harm has been done by the sources 

 of error peculiar to each plate, which would tend to make the results 

 from the separate plates disagree. Among these sources of error 

 may be mentioned irregular distortion of the films, peculiar imper- 

 fections of the photographic images, and possible varying obliquity 

 in the mounting of the plates, with respect to the optical axis of the 

 telescope. The actual probable errors of the final co-ordinates are 

 of course larger than those just given, since they involve also the 

 remaining uncertainty of the scale value, as well as certain other 

 possible sources of error. There does not seem to be any very 

 marked difference in the probable errors for the stars of various 

 magnitudes. That the probable errors of the position angles ex- 

 ceed those of the distances, may be due to the more absolute char- 

 acter of the former (see also Section VI.). It should perhaps be 

 mentioned that every observation contained in the Rutherfurd 

 observation books has been employed in the reductions ; with the 

 exception of one, which was found to be erroneous upon compari- 

 son with the original negatives. 



In conclusion, acknowledgment is made, on the part of Mr. 

 Rutherfurd, to Miss Ida C. Martin, who executed the measures 

 under his direction. Mr. D. C. Chapman was Mr. Rutherfurd's 

 observatory assistant during the period of the Pleiades observa- 

 tions. 



