MOUNT WILSON OBSERVATORY. 231 



THE NATURE OF SUN-SPOTS. 



In an illustrated address delivered before the Royal Society on 

 November 7, 1918, the Director gave a comprehensive sketch of his 

 researches on the nature of sun-spots. This address, which will be 

 published in full in the Philosophical Transactions, comprised (1) an 

 historical introduction, briefly summarizing the views on sun-spots of 

 Galileo, Derham, Alexander Wilson, Sir William Herschel, Sir John 

 Herschel, Faye, Secchi, Young, Halm, and Emden, with special 

 reference to the vortex theory, first mentioned as a possibility by 

 Wilson and outlined, after the model of terrestrial cyclones, by Sir 

 John Herschel ; (2) a description of the development of the instruments 

 and methods used in the present work, from the Kenwood spectro- 

 heliograph, employed also in 1891 as a spectrograph for the photography 

 of sun-spot spectra, to the Rumford spectroheliograph of the 40-inch 

 Yerkes refractor, the 5-foot spectroheliograph and 18-foot spectrograph 

 of the Snow telescope, the 13-foot spectroheliograph and 30-foot 

 spectrograph of the 60-foot tower telescope, and the 75-foot spec- 

 trograph of the 150-foot tower telescope; (3) the successive results 

 obtained in photography of the solar atmosphere with the spectro- 

 heliograph, including the prominences at the limb, the calcium flocculi 

 on the disk at various levels, and the bright and dark hydrogen flocculi, 

 indicating the effects of absorption at still higher levels and showing 

 prominences as dark objects projected against the disk; (4) the dis- 

 covery, on photographs made with the Ha line, of vortex phenomena 

 centering in sun-spots, and the chief characteristics of these vortices; 

 (5) the formulation of the hypothesis that a sun-spot is an electric 

 vortex, the detection of the Zeeman effect due to the resulting magnetic 

 field, and the observation of the field-strength, inclination of the lines 

 of force, and other magnetic phenomena; (6) the discovery of bipolar 

 spots, the development and application of a scheme of classifying 

 sun-spots in accordance with their magnetic polarities, and the recog- 

 nition of the leading characteristics of the law of solar storms; (7) the 

 observation of strengthened, weakened, and band lines in the spectra 

 of sun-spots, and the proof they afford of reduced temperature; (8) the 

 work of Evershed and St. John on the nature of the spot vortex; (9) 

 the results of vortex and other experiments bearing on the nature of 

 sun-spots and the associated flocculi; (10) the negative results of pre- 

 liminary attempts to detect electric fields in sun-spots; and (11) the 

 discovery of the general magnetic field of the sun and its bearing on 

 the nature of sun-spots. 



PHOTOGRAPHS OF THE SUN WITH ULTRA-VIOLET LIGHT. 



In photographing the spectra of sun-spots, it is necessary to give 

 longer and longer exposures to the umbra in passing from the red to 

 the violet. A similar increase of general absorption with decreasing 



