MOUNT WILSON SOLAR OBSERVATORY. 207 



general field of the sun have hindered progress in this investigation. 

 The delay is not serious, however, as much better photographs can 

 probably be obtained when the sun is more active than at present. 



Radial Motion in Sun-spots. 



Mr. St. John's investigations on the radial motion of the vapors 

 in the solar atmosphere above sun-spots have been greatly extended 

 during the year. In a paper published as No. 69 of the Observatory 

 Contributions he has given his results on the distribution of velocities 

 in the solar vortex, together with a discussion of various phenomena 

 connected with the theory of sun-spots. The program of observa- 

 tions has included the measurement of radial velocities for 506 lines 

 representing 27 elements, lying at levels extending from the lowest 

 that can be observed to the highest regions of the chromosphere. The 

 method of observation employed has yielded results of high preci- 

 sion, which maybe depended upon to represent the facts very closely 

 indeed. 



An examination of the absolute displacements shows that the large 

 displacements are associated with the longer wave-lengths, as must be 

 the case if they are really due to motion of the vapors in the line of 

 sight. Another important fact is the systematic variation of the dis- 

 placements with the intensities of the Fraunhofer lines. The mean 

 displacements of 193 iron lines of intensities 1 to 8, when reduced to 

 a common wave-length, show a gradual decrease in velocity from 

 1.68 km. per second for lines of intensity 1 to 0.54 km. per second for 

 lines of intensity 8. If the series of iron lines be extended so as to 

 include those from intensity 00 to the very strong lines of intensity 

 15 to 40, thus covering the entire range, an iron scale is provided 

 which forms a practical and convenient means of determining the 

 relative levels in the solar atmosphere at which the lines of other 

 elements are produced. For these lines the flow of the gases is out- 

 ward and tangential to the solar surface, and if, as the evidence 

 indicates, the weaker lines originate at the lower level, the velocity out- 

 ward increases with the depth. The results also show that the mean 

 level of the lines in the red is about two units of intensity lower than 

 in the violet; that is, lines of intensity 4 in the red are at about the 

 level of lines of intensitj^ 2 in the violet, and similarly for lines of 

 other intensities. Such a difference is the natural consequence of 

 the scattering of light bj- small particles, which varies inversely as 

 the fourth power of the wave-length. From this it would result that 

 we see into the sun to a greater depth in the red than in the violet. 



The distribution of velocities in sun-spots is shown by the accom- 

 panying figure, which represents a vertical section through the axis 

 of the vortex. In the lower portion, corresponding to the effective 

 levels of the reversing layer, the velocities are mainlj^ those given by 

 the iron lines; in the upper portion, which refers to the chromosphere, 



