MOUNT WILSON SOLAR OBSERVATORY. 1 51 



two positions of the sun's image on the slit, a special attachment has been 

 constructed for the purpose. Two small diagonal prisms throw light from 

 the edge of the sun upon the slit, while the light from the center passes un- 

 obstructed on either side of the central prism. Photographs taken with this 

 apparatus have proved extremely satisfactory and thoroughly reliable as re- 

 gards the absolute displacements involved. At the present time negatives 

 are being taken for the purpose of providing a photographic map of the 

 spectra of these two portions of the sun's disk, and it is expected that this 

 map will be completed some time during the winter. 



In addition to the conclusions drawn from the measurement of the dis- 

 placements referred to in the last report, the important result has been found 

 that the spark or "enhanced" lines in the spectrum seem to show greater dis- 

 placements at the limb of the sun than do the majority of the other lines. 

 This has a vital bearing on the question of the level at which these lines orig- 

 inate in the sun's atmosphere. It will require, for satisfactory explanation, 

 a series of laboratory investigations on the displacements of spark lines under 

 pressure. 



Spectrographic Investigation of the Solar Rotation. — Mr. Adams's photo- 

 graphic investigation of the rotation of the sun, by means of the displace- 

 ments of the spectrum lines at the opposite limbs, has been continued 

 throughout the year with the powerful spectrograph of the tower telescope. 

 The advantages of this instrument over the i8-foot spectrograph of the Snow 

 telescope, with which the investigation of 1906 to 1907 was carried on, are 

 very considerable. In addition to the superior quality of the solar image, 

 and greater freedom from astigmatism and change of focus, the larger linear 

 scale furnished by the instrument, and the possibility of setting for different 

 position angles, by rotation about a vertical axis, have proved of great value. 

 The plates employed in the study of the rotation from lines in the violet part 

 of the spectrum have all been taken in the third order of the grating. Plates 

 including the region about Ha in the red part of the spectrum have been 

 taken in the second order. 



About 25 plates covering the same region of the spectrum that was em- 

 ployed in the investigation of last year have been measured and reduced. 

 The degree of accuracy obtained seems to be appreciably higher than that 

 secured with the plates from the 18-foot spectrograph. The comparison of 

 the results of the two series of observations indicates very close agreement in 

 the latitudes running from 0° to 45°. From 60° to the pole, however, the 

 values obtained from the present series fall appreciably below those of last 

 year, the difference amounting to about 0.04 km. at a maximum. It does not 

 at present seem probable that this difference is to be ascribed to a real varia- 

 tion in the rotation of the sun, since it is not shared in by the zones of lower 

 latitude, in which the activity of the sun, as indicated by the presence of spots 



