262 CARNEGIE INSTITUTION OF WASHINGTON. 



merits between the east and west limbs for some typical lines have been 

 measured by means of curves obtained with the Koch microphotometer. 

 The preliminary results from 6 plates for three lines of intensity 2 and 

 the two magnesium lines X 5172 and X5183 show systematic differences 

 in displacement of the same order as those shown by the filar microm- 

 eter measurements. The displacements in millimeters found by the filar 

 micrometer are given in the upper line, while those determined from 

 measurement of the curves made by the microphotometer appear in 

 the lower line of the summary. The data refer to the same plates. 



X5165 X5225 \5226 Mean. X5172 X5183 Mean. 



0.0842 0.0861 0.0858 0.0853 0.0892 0.0894 0.0893 mm. 

 0.0852 0.0859 0.0853 0.0855 0.0886 0.0878 0.0882 



When the equatorial velocities obtained from the displacements of 

 the unenhanced lines are grouped according to their intensity, a system- 

 atic variation is very apparent. 



Intensity 1.0 2.0 3.6 6.5 22 



Number of lines 2 6 5 4 3 



Equatorial velocity 1.924 1.933 1.937 1.954 2.043 



The equatorial velocities for enhanced lines are slightly lower 

 than for unenhanced lines of the same mean intensity, a result in 

 harmony with that from the two earlier investigations. For 6 enhanced 

 lines of mean solar intensity 3.2, the rotation value is 1.928 km., while 

 for 5 unenhanced lines of mean intensity 3.6, it is 1.937 km. 



From the 1906-07 series of Mount Wilson observations, the equa- 

 torial velocity found was 2.064 km., while that from the 1908 series 

 was 2.053 km. In view of the still lower value obtained from the cur- 

 rent series (1.949 km.), and the generally low values found by recent 

 observers at epochs falling between the two last Mount Wilson series, 

 the question of the constancy of the solar rotation may be seriously 

 raised. This systematic decrease in the values found for the solar 

 rotation emphasizes the need of entering upon a series of observations 

 extending over a considerable period of time. Such a series has been 

 begun, and it is intended to obtain plates in zero latitude under the 

 same observing conditions each month for some years to come. 



A more extended series of measurements based upon the curves 

 obtained by the microphotometer is under way. If the preliminary 

 results for the magnesium lines are confirmed, the method will be 

 employed for a larger number of lines. The method appears eminently 

 fitted to determine whether the variations found by the filar micrometer 

 are real or are occasioned by the personal characteristics of the observer. 



