226 CARNEGIE INSTITUTION OF WASHINGTON. 



The recent marked decrease in the number of spots has permitted 

 Mr. Ellerman to make a long series of photographs of spectra for the 

 purposes of another determination of the magnetic elements of the sun. 



SOLAR ROTATION. 



A series of observations of the solar rotation, made with the 75-foot 

 spectrograph of the 150-foot tower telescope under uniform con- 

 ditions, was begun at Mount Wilson in 1914. Taken year by year, 

 the period of rotation has proved to be practically constant. The sun 

 is now approaching the period of minimum spot-activity, when changes 

 in rotation associated with the sun-spot cycle should become manifest, 

 if such exist. The next few years ought to furnish definitive data. 



An improvement in the centering of the light-cone upon the grating 

 is obtained by placing a 2-cm. diaphragm on the center of the telescope 

 objective. The beam incident upon the grating is easily made rigor- 

 ously central by adjusting the prisms for each exposure until the 

 image of this spot is central upon the grating. An investigation is 

 being made of the effects produced by more or less faulty centering, 

 still keeping the illumination of the grating surface practically uniform 

 when judged by the eye. 



The completion of the Snow telescope equipment furnishes an 

 opportunity for obtaining simultaneous observations with it and the 

 150-foot tower telescope, with which the long series is taken. It is 

 hoped by this means to discriminate between effects due to solar 

 causes and those due to instrumental and observing conditions. 



GENERALIZED RELATIVITY. 



The results of the British observations at the solar eclipse of 1919 

 have greatly increased the interest in the theory of generalized rela- 

 tivity. Of the three instances in which the deductions from Ein- 

 stein's modified law of gravitation indicate effects within the range 

 of observation, viz, the anomaly of Mercury's perihelion, the deviation 

 of light in passing through the sun's gravitational field, and the gravi- 

 tational displacement of the solar lines toward longer wave-lengths, 

 the last presents the main outstanding discrepancy between theory 

 and observation. In the opinion of Professor Einstein and the sup- 

 porters of the theory of generalized relativity, a displacement of all 

 solar lines to the red is a necessary and fundamental condition for its 

 acceptance. A definitive result would be of great importance and is 

 greatly to be desired from all points of view. In the annual report for 

 1917 the results of an investigation by Mr. St. John of the behavior of 

 lines in the cyanogen band at X3883 were given. These were unfavor- 

 able to the Einstein hypothesis. To this negative evidence may now 

 be added that shown by the magnesium triplet in the green: 



Center sun 5,167 . 336 a. .5,172 . 699 a. 5,1S3 . 619 a. 



Reversed arc 5,167. .336 5,172.696 6,183.618 



Sun-arc 0.000 +0.003 +0.001 



Limb -center +0.002 +0.001 +0.001 



