MOUNT WILSON SOLAR OBSERVATORY. 



185 



Radial Motion in Sun-spots. 



Additional plates have been taken during the year by Air. St. John in the 

 course of his investigation of the radial motion in sun-spots. These have all 

 been measured, yielding results in harmony with those previously obtained, 

 which indicate the following mean conditions : 



The calcium vapor as represented by the H and K lines and the hydrogen 

 as shown by the Ha line flow inward across the outer boundary of the pe- 

 numbra with velocities of 1.9 km. and 1.5 km. per second, respectively; the 

 vapors of sodium and magnesium flow inward with velocities of 0.2 km. 

 and 0.4 km. per second, respectively ; the vapors of aluminum and iron, 

 which are the sources of the strong Al line A 3961, intensity 20, and of the 

 Fe line A 4271, intensity 15, have mean velocities of 0.0 km. and 0.1 km. per 

 second inward respectively. The mean provisional results for iron, based 

 upon a large number of lines of low and moderate intensities near A 5200, 

 are shown in the table. 



Similar results appear in the case of other metallic va- 

 pors producing lines of low and moderate intensities, though 

 the velocities deduced from lines of equal intensities for 

 different elements are not equal. This offers a means of 

 determining the relative distribution of the constituents of 

 the solar atmosphere or the relative levels at which lines 

 of given intensities have their origin, as referred to some 

 standard, such as iron, represented by a large number of 

 lines in the solar spectrum. For example, it appears that 

 the lines of titanium are produced at a higher level than lines of the same 

 intensity due to iron. 



Three regions are indicated in the solar atmosphere surrounding spots, 

 with the following characteristics at the different levels : 



(1) The upper chromosphere, in which the movements are directed in- 

 ward, as shown by the H and K lines of calcium and the Ha line of 

 hydrogen. 



(2) An intermediate region, in which are produced the D lines of sodium, 

 the b lines of magnesium, and the strongest lines of aluminum and iron, in 

 which the movement is sometimes inward and sometimes outward, the in- 

 ward direction in general prevailing. 



(3) A lower region, in which the movement is always directed outward, 

 the velocity increasing with the depth or nearness to the photosphere, on 

 the assumption that in general the weaker lines are associated with the lower 

 levels. 



THE SIXTY-INCH REFLECTOR. 



During the year a new extension frame for the end of the tube of the 

 60-inch reflector has been completed, which admits of photographic work 

 directly in the axis of the large mirror and thus eliminates one reflecting 

 surface. Three spectrographs for use with this frame are available : 



