REPORT OF THE SECRETARY. 71 



must be hot. As for the idea of luminescence by electrical discharge, though 

 the streamers of the corona are a reminder of the aurora, one hesitates to rec- 

 ommend an explanation involving a thing so little understood, so that we will 

 here speak only of the incandescence and reflection of the corona as sources of 

 its brightness. The bolometric results indicate that the coronal radiation dif- 

 fers but little in quality from that of the sun, and is, iu fact, far richer than the 

 reflected rays of the moon in visible light, although less rich than skylight. 



These results indicate that if produced by virtue of high temperature, the 

 coronal radiation must have come from a source almost as hot as the sun, which 

 is upward of 6,000° absolute. Such temperatux'es as this are too high for the 

 existence of any known solids or liquids, unless under high pressures not found 

 in the corona, so that if the light is due to the high temperature of the corona 

 itself, the corona must apparently be gaseous. But if it is gaseous, its spec- 

 trum should consist chiefly of bright lines, and this is not the fact. Hence it 

 would seem that the coronal radiation, if it is produced by temperature, has its 

 source in the sun itself, and is merely reflected by the matter of the corona, 

 like the light of our atmosphere. But if the coronal rays are reflected, they 

 would be bluer than sunlight, if the material there is gaseous, and as they are 

 not, the coronal material may be supposed to be composed of solid or liquid 

 particles to a considerable extent. But it is objected that only the outer corona 

 shows the characteristic dark lines of the solar spectrum, and that these are 

 absent in the region of the corona now being considered. May it not be that 

 the temperatui-e of the inner corona is so high that gases are present there 

 along with the solid and liquid particles, so that the bright-line spectrum of 

 these gases may be present and be superposed upon the reflected solar spec- 

 trum? In this case the bright rays of incandescence would fall exactly upon 

 the dark lines of the solar spectrum and tend to obliterate them. At points in 

 the corona more remote from the sun the gases would cool to liquid drops, or 

 solid particles, or become excessively rare, so that the bright-line spectrum of 

 incandescent gas would fade away, leaving the dark lines of the x-eflected solar 

 spectrum pi-edominant. 



This line of explanation seems to me to accord with the facts observed, but 

 I give it merely as a suggestion. 



4. MOUNT WILSON OBSERVATIONS. 



Great advantage having been found in 190.5 and 1906 in making " solar-con- 

 stant " investigations on Mount Wilson as well as iu Washington, and strong 

 evidence having been secured there of the considerable variability of the sun, 

 it was concluded to continue iu 1908 the expedition to Mount Wilson iu order 

 to secure as many observations of the " solar constant " as possible for the study 

 of solar changes. As in former years, other kinds of measurements were con- 

 templated, notably on the brightness of the sky and on the reflection of the 

 clouds. The expedition, iu charge of the writer, and including also Mr. L. B. 

 Aldrich, of Madison, Wis., reached Mount Wilson on May 11, 1908. " Solar- 

 constant " observations were begun on May 19 and have been made ever since 

 daily when the sky permitted. Unfortunately the sky has been less clear on 

 Mount Wilson than in other years, but nevertheless a great number of observa- 

 tions have been made. 



The apparatus employed for observing the eclipse on Flint Island has been 

 erected on Mount Wilson on the tower built for the Smithsonian expedition in 

 1906, and wuth this apparatus some of the observations of the brightness of the 

 sky have already been made. It is expected to continue these measurements 

 and others on the reflecting power of the clouds during the stay of the expe- 

 dition. 



88292— SM 1908 6 



