[PLASKETT-DELURY] THE SOLAR ROTATION 59 
means of detecting such an error would be by the comparison of spectra 
made at the same epoch by different instruments and methods and 
measured by the same observer, but such is not easy to arrange. The 
differences in value for successive plates taken under so far as known 
identical conditions (previously referred to in Section 16) is most likely 
due to some sort of instrumental error unless rapid changes in local 
motions in the reversing layer are responsible. Although these dif- 
ferences are apparently quite accidental they may nevertheless contain 
a small systematic deviation. 
(c) Personal Errors of Measurement.—It has been shown (Sections 
15-17) that it is possible, even probable, for such differences as those 
in question to be obtained on measurement of the same plate by different 
observers, and it seems useless to consider other sources of error until 
it is possible to eliminate this. Although the difference between 
Plaskett and De Lury is fairly well determined at À 5600 as at present 
about 0.040 km. per second, sufficient plates in common have not yet 
been measured to determine the difference between Miss Lasby, by 
whom most of the Mt. Wilson plates were measured, and the writers. 
Her measures appear to be somewhat higher on the whole (Section 17) 
than Plaskett’s, and the same tendency was shown even more markedly 
during a visit of the latter to Mt. Wilson in 1910, where comparisons of 
the measured displacements of several lines on rotation plates at the 
equator showed that Miss Lasby’s measures were always two or three 
per cent. higher than Plaskett’s. If there is this difference, then the 
actual velocity displacements on the Mt. Wilson and Ottawa plates 
are approximately the same, and it only remains to determine whose 
measurement is the most nearly correct. At present, however, we will 
have to be satisfied with recognizing the presence of personal differences 
of measurement, as accounting for part at any rate of the differences 
in velocity obtained. 
25. In view of these actual differences of velocity obtained by 
the different observers and after the discussion of the probable causes 
of these differences, we can only state that the velocity of the solar 
rotation as determined from Plaskett’s measurements is represented by 
the formulæ 
V = (1.483 + 0.532 cos’ 9) cos ¢ 
£ = 10°.32. == 47:00" cos) 
and that this angular formula differs from Adams’ 1908 formula practi- 
cally only in the constant term and is also in good agreement with 
Dunér’s, and that hence it probably represents very closely the relative 
velocities at the different latitudes, although the absolute values may be 
uncertain by, say, 2 per cent. 
