134 RElPORTS Oif INVESTIGATIONS AND PROJECTS. 



MOUNT WILSON SOLAR OBSERVATORY * 



George E. Hale, Director. 



Current views of solar phenomena are based on the assumption that light 

 from all parts of the sun's disk reaches us along nearly straight lines. So 

 far as white light is concerned this assumption may be warranted, if, as is 

 generally believed, the pressure of the solar atmosphere is low. But in the 

 case of monochromatic light, differing but slightly in wave-length from any 

 of the dark lines of the solar spectrum, the influence of anomalous dispersion 

 should be taken into account. In its passage through the gases and metallic 

 vapors of the solar atmosphere, such light must deviate from a straight path 

 by an amount which depends upon the density gradient of the vapor pro- 

 ducing the corresponding Fraunhofer line. Even for slight density gra- 

 dients, smaller than those ordinarily assumed to exist in the chromosphere 

 and reversing layer, the effects of anomalous dispersion have been found by 

 laboratory experiments to be very striking indeed. We must therefore seek 

 to determine in what degree our interpretations of solar phenomena require 

 modification in the light of this principle. 



We are indebted to Professor Julius, of the University of Utrecht, for a 

 series of important papers, in which many of the possible appHcations of 

 anomalous dispersion to solar theory are set forth in detail. According to 

 his views, which are based upon laboratory investigations, the chromosphere 

 and prominences do not exist as they appear to us ; the bright lines of their 

 spectra are not the radiations of luminous gases, but photospheric light 

 brought into our line of vision after anomalous refraction through non- 

 homogeneous gases. Such effects, when imitated experimentally in the labo- 

 ratory, are very striking. But astronomers are not yet read)^ to admit that 

 chromosphere and prominences, faculce and flocculi, and other phenomena 

 long universally held to have objective existence, are simply due to anom- 

 alous ray-curving. They demand far more evidence than has yet been sup- 

 plied. Physicists, on the other hand, are more favorable to Professor 

 Julius's views. Unhampered by that impelling belief in solar phenomena 

 which results from decades of observation, they ask how the phenomena of 

 anomalous dispersion can possibly be absent, under conditions admittedly so 

 favorable for their manifestation. 



It was thus a fortunate circumstance that permitted Professor Julius to 

 spend some wxeks on Mount Wilson during the past summer. His labo- 

 ratory in Utrecht is devoted entirely to physical work, and is not equipped 

 for solar investigations. For this reason our collection of solar photographs, 

 and the opportunity to use the spectroheliograph and other instruments, was 



* Situated on Mount Wilson, California. Grant No. 403. $85,000 for construction, 

 investigations, and maintenance. (For previous reports see Year Book No. 3, pp. 154- 

 174; Year Book No. 4, pp. 56-77; and Year Book No. 5, pp. 60-86.) 



