RECENT ADVANCES IN SOLAR ASTRONOMY. 31 



At the eclipse of 1883, observed on Caroline Island, in the Pacific 

 Ocean, by French and American parties, Professor Hastings made ob- 

 servations for the purpose of testing a theory he had framed that the 

 outlying regions of the corona are merely a diffraction effect pro- 

 duced by the edge of the moon ; the diffraction being not that due to 

 the regular periodicity of light-vibrations, ordinarily discussed, but 

 due to the probable continually occurring discontinuity or change of 

 phase in the vibrations. It seems probable, from a not perfectly com- 

 plete investigation, that such discontinuity might scatter light far be- 

 yond the limits of ordinary diffraction. He found during the eclipse, 

 by an apparatus constructed expressly for the purpose, that the bright 

 corona-line (1474 K) was always visible to a much greater distance 

 from the sun on the side least deeply covered by the moon than on the 

 other, as unquestionably ought to be the case if his theory were correct. 



But the same thing would result from the diffusion of light by the 

 air ; and the French observers, and nearly all others who have dis- 

 cussed the matter, feel satisfied that this is the true explanation of 

 what he saw. He himself now, we understand, thinks it not impossi- 

 ble that a thin cloud may have passed over the sun just at the critical 

 moment, and so have vitiated his observation. 



The discussion which has followed his publication seems to have 

 only strengthened the older view, that the corona is a true solar 

 appendage, an intensely luminous though inconceivably attenuated 

 cloud of gas, fog, and dust, surrounding the sun, formed and shaped 

 by solar forces. 



The fact that comets, themselves mere airy nothings, have several 

 times (the last instance was in 1882) passed absolutely through the 

 corona without experiencing any sensible disturbance of path or 

 structure, has, however, been always felt by many as an almost in- 

 superable difficulty with this accepted theory, and more than anything 

 else led Professor Hastings to propose his new hypothesis. But, on 

 careful consideration, we shall find that our conceptions of the possible 

 attenuation of shining matter near the sun will bear all the needed 

 stretching without involving any absurdity. Recalling the phenomena 

 of the electrical discharge in Crookes's tubes, it is clear that a " cloud," 

 with perhaps only a single molecule to the cubic foot (but thousands of 

 miles in thickness), would answer every luminous condition of the phe- 

 nomena. And all the rifts and streamers, and all the peculiar struct- 

 ure and curved details of form, cry out against the diffraction hy- 

 pothesis. 



At present the most interesting debate upon the subject centers 

 around the attempts of Dr. Huggins (first in 1883) to obtain photo- 

 graphs of the corona in full sunlight. He succeeded in getting a num- 

 ber of plates showing around the sun certain faint, elusive halo-forms 

 which certainly look very coronal. Plans were made, and were car- 

 ried out, in 1884, for using a similar apparatus upon the Riffelberg 



