472 Scientific Intelligence. 



imperfections in the micrometer screw or progressive change in 

 its scale value, must impress the reader as original, judicious and 

 comprehensive ; and as the value of the parallax work depends 

 chiefly on the completeness of these eliminations, the inferences 

 drawn by the authors as to the sigificance of their results may be 

 accepted with great confidence because of the high order of this 

 part of the work. 



An interesting example of the thoroughness of the investiga- 

 tion of sources of systematic error is the discussion by Dr. Chase of 

 the effect of difference of color between the parallax star and its 

 star of comparison. It is assumed that the light of a red star 

 being refracted less than that of a white comparison star, the 

 angular distance between them will be affected. 



The treatment of this topic is of such intrinsic interest and so 

 typical of the thoroughness of the whole investigation of sources 

 of systematic error that it seems well to reprint it here entire 

 from the text of the report — the tables of measurements, occupy- 

 ing many pages, being omitted. 



" Observations of red stars for color effect. — The work of the 

 preceding pages, in which every precaution to eliminate known 

 sources of error was employed, appears to us to be free from 

 all systematic error, except perhaps one due to the star's color. 

 Any perceptible difference in the mean refrangibility of the light 

 of two stars might possibly produce an effect upon the meas- 

 ured distance between them which would be a function of 

 the hour angle, and hence affect their apparent relative par- 

 allax, since it is generally impossible for practical reasons with 

 our instrument to make observations at the two different 

 parallax epochs otherwise than on opposite sides of the meridian. 

 To ascertain whether an error due to this cause is appreciable in 

 actual observation, Dr. Chase, in 1898 and 1899, made a series of 

 observations on five highly colored stars selected from Krtiger's 

 Catalog der Farbigen Sterne, carrying out the investigation in 

 the following manner : 



The plan was at the epoch when the star culminates about 

 midnight to measure the distances between the red star and each 

 of two nearly equally distant comparison stars, one preceding 

 and the other following, and as nearly as possible on the same 

 parallel of declination, at rather large hour angles both east and 

 west of the meridian. By taking two comparison stars we were 

 able, as in the parallax work, to correct for any change in the 

 scale value, and to eliminate any errors varying with the time. 



The refraction of two stars with light of a different mean 

 refrangibility being represented by 



ft tan z, and (/3 + A/3) tan z 



the measured distance between two such stars should receive, 

 besides the correction for differential refraction, the additional 

 correction 



A/3 tan z cos (p — q) 



