REPORT OF THE SECRETARY. 69 



3. SOLAE ECLIPSE EXPEDITION — A BOLOMETRIC STUDY OF THE SOLAR CORONA. 



The Smithsonian Institution was represented among observers of the eclipse 

 of January 3, 1908, by a small expedition including the writer and Mr, A. F. 

 Moore, of Los Angeles. Our charges were defrayed by the Institution, but we 

 went by invitation and with the cooperation of Director Campbell, of the Lick 

 Observatory, and shared in the benefits of the careful provision which he made 

 for the general welfare and success. The observations were made at Flint 

 Island in the South Pacific Ocean. My absence from Washington extended from 

 November 5, 1907, to February 2, 1908. 



We proposed to measure, with that extremely sensitive electrical thermometer 

 called the " bolometer," the intensity of the radiation of the solar corona, and to 

 determine the quality of coronal light as compared with sunlight. 



In the year 1900 the first bolometric observations of the corona were made 

 by Smithsonian observers. From these observations it was inferred that, as 

 regards quality, the radiation of the inner corona was far richer than that 

 refiected from the moon in visible light. In view of this consideration and 

 others, the inferences drawn by the writer from the bolometric study of the 

 corona made in 1900 were unfavorable to the view that the radiation of the 

 inner corona is produced mainly by the incandescence of matter heated to high 

 temperatures by reason of its proximity to the sun. The bolometric observa- 

 tions at Flint Island were designed to test the inferences above referred to and 

 to measure more definitely the quantity and quality of the coronal radiation. 



Apparatus. — A concave mirror of 50 centimeters diameter and only 100 centi- 

 meters focus, mounted equatorially and driven by a clock, served to produce a 

 very intense image of the corona. A small guiding telescope was attached to the 

 mirror frame so that the observer might point toward any desired object. In 

 the focus of the mirror was placed the bolometer. A glass plate 3 millimeters 

 thick was fixed close to the bolometer, between it and the mirror, so that the 

 radiation examined was thereby limited to wave-lengths less than about S^i. 

 About 10 centimeters in front of the bolometer was a blackened metal shutter, 

 which cut off the beam except when designedly opened. The opening of this 

 shutter, therefore, exposed the central part of the bolometer to such rays as 

 are transmissible by glass. Between the shutter and the glass plate, and close 

 to the latter, was a special screen composed of a thin stratum of asphaltum 

 varnish laid on one side of a plane parallel glass plate 3 millimeters thick. 

 This screen was held out of the beam by a spring, except when designedly inter- 

 posed. Its property, when used, was to cut off nearly all the visible part of 

 the radiation, while transmitting nearly all of the infra-red rays transmissible 

 by glass. By interposing this absorbing screen the proportion of the observed 

 radiation which lay in the infra-red spectrum could be roughly determined. 



The equatorial was set up at Flint Island on the beach at about 12 meters 

 distance from the galvanometer used for observing the indications of the bolom- 

 eter. Two galvanometers were provided, exactly alike in resistance and general 

 construction, and arranged so that if at the last moment any accident should 

 happen to one the observer might pass at once to the other.*^ A thatched hut, 

 shaded by palm trees, sheltered the galvanometers and their appliances, and 

 was found to give most satisfactory protection both from heat and rain. During 

 the eclipse a rise of temperature of one bolometer strip of about 0°.000,01 C. 

 would have produced 1 millimeter deflection of the galvanometer. It is possible 

 to detect temperature changes of 0°. 000,000,01 C. with the bolometer, under 

 special conditions, but the sensitiveness employed was regarded as good for a 

 temporary installation. 



"This prudent measure was suggested by Mrs. Abbot. 



