. The Astrophysical journal. the mean of the velocities at z<i and u.; this diflfer-ence is very closely equal to three times the correction, the errorbeing always less than ,XU2 — U^y M2+M1 K——— COS . 2880 2 This approximation lends itself readih* to graphical the diagram let IlFa be a portion of the true velocity-curve,Ti corresponding to ih, and V. to U2. From the middle point of thechord F1F2 erect the vertical AB; the correction is equal to one-third oi AB. I TO .U/.VOA CilXTKIBLTIOXS AM) XOTES We have confined our attention to circular orbits. There wouldbe no particular
. The Astrophysical journal. the mean of the velocities at z<i and u.; this diflfer-ence is very closely equal to three times the correction, the errorbeing always less than ,XU2 — U^y M2+M1 K——— COS . 2880 2 This approximation lends itself readih* to graphical the diagram let IlFa be a portion of the true velocity-curve,Ti corresponding to ih, and V. to U2. From the middle point of thechord F1F2 erect the vertical AB; the correction is equal to one-third oi AB. I TO .U/.VOA CilXTKIBLTIOXS AM) XOTES We have confined our attention to circular orbits. There wouldbe no particular ditliculty in extending the present results to includeelliptic orbits, but the necessity for this has not yet arisen, and thereis reason to believe that it cannot arise, the orbits of all short-period binaries being very nearly circular. It is worthy of remarkthat the graphical correction described in the preceding paragraphwould apply nearly as well to orbits of moderate eccentricity as tocircular Long exposures not only cause the systematic error that wehave discussed, but they also tend to increase accidental error in anumber of ways: (i) changes of temperature in the spectrographare hkely to be greater; (2) differential flexure is greater; (3) theeffective mean time of exposure may be uncertain by reason ofchanges in the transparency of the atmosphere and in the seeing;(4) if the spectrogram is exposed near the times of maximum orminimum velocity, the lines may be made asymmetric, and theobserver may have considerable difhculty in deciding upon whatportion of the line to make the micrometer settings; this errorwould probably vary to considerable extent with the strength ofthe spectrogram and with the manner in which it was developed. Frank Schlesinger Allegheny Observatory University of Pittsburgh January 21, 1916 MINOR CONTRIBUTIONS AND NOTES 171 NOTE ON TERRESTRIAL RADIATION The radiation from the surface of the earth which does notescape immediately to
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