3 o2 POPULAR SCIENCE MONTHLY. 



3. A small proportion of the inner, and a large proportion of the 

 outer, coronal light are solar rays reflected and diffracted by the coronal 

 particles. 



Arrhenius has recently shown that Abbot's observation of an ap- 

 parent temperature of the corona nearly equal to that of his observing 

 room is in harmony with the spectrographic evidence of an inner 

 corona composed of incandescent particles. Arrhenius finds that one 

 minute dust particle to each 11 cubic meters of space in the coronal 

 region observed by Abbot, raised to the temperature of 4620° absolute 

 required by Stefan's law, would give a corona of the observed bright- 

 ness, and of the observed temperature. The bolometric strip meas- 

 ured the resultant temperature of the few highly-heated particles and 

 the cold background of space upon which the particles are seen in pro- 

 jection. 



Arrhenius further estimates that a corona composed of incandescent 

 dust particles need not have a total mass greater than 25,000,000 tons, 

 to radiate the quantity of light yielded by the brightest corona observed. 

 This is approximately that of a cube of granite only 200 meters on 

 each side; a remarkably small mass for a volume whose linear dimen- 

 sions are millions of kilometers. 



This resume of solar theory necessarily overlooks many unsettled 

 points of great significance. Most important of all, perhaps, is that 

 of the solar constant: does it vary, and in accordance with what law? 

 Why is there a sun-spot period, and why are the large spots grouped 

 within limited zones? Why does the form of the corona vary in a 

 period equal in length to the spot period? Why does the angular 

 speed of rotation increase from the poles to the equator? What is the 

 origin of the faculae and the flocculi? Why do the Fraunhofer lines 

 show little evidence of high atmospheric pressure? Why are the 

 radiations from calcium, one of the heavy elements, so prominent in 

 the higher chromospheric strata and in the prominences? A great 

 number of such questions are pressing for solution. Under the 

 stimulus of the brilliant researches of our chairman, the reinventor 

 and the leading developer of the spectroheliograph, cooperative plans 

 for solar work on a large scale are now being organized. We should 

 be vitally interested in promoting these plans; for the study of the 

 sun, as the principal foundation of astrophysical research, has been 

 unduly neglected. 



The celestial bodies develop under conditions over which we have 

 no control. We must observe the facts as they are, at long range, and 

 interpret them in accordance with those principles of physical science 

 which govern what seem to be closely related terrestrial phenomena. 

 A successful study of the development of matter in distant space, under 

 the influence of heat, pressure, electricity and other forces of nature 



