118 



Dr. W. Huggins. 



the image has been formed by a lens which shortens and enfeebles 

 the ultra-violet part of the light, or by a mirror which furnishes an 

 image more truly representing the corona in the nature of the light 

 existing in it.. The difficulties which seemed to lie in the way of a 

 satisfactory explanation of the forms and of the enormous extent of 

 the corona, caused some doubts to be entertained as to the corona 

 being a true solar appendage, and various views were formerly put 

 forward to> endeavour to explain the corona as an optical appearance 

 only, arising from our atmosphere, from a lunar atmosphere, or from 

 cosmical dust. Mr. De la Rue, in his address before Section A of the 

 British Association in 1872, says truly, " The great problem of the 

 solar origin of that portion of the corona which extends more than a 

 million of miles beyond the body of the sun, has been, by the photo- 

 graphic observations of Colonel Tennant and Lord Lindsay in 1871, 

 finally set at rest, after having been the subject of a great amount of 

 discussion for many years.."* 



These earlier views are too completely a part of the history of the 

 subject to need mention here, but for the circumstance that Professor 

 Hastings has recently revived the theory of Delisle, that the corona is 

 an optical appearance duo to diffraction. 



Professor Hastings bases his theory upon the behaviour of the 

 bright line, 1474, which he saw, in his spectroscope, change in, length 

 east and west of the sun during the progress of the eclipse at Caro- 

 line Island in 1883. He assumes, in his explanation of this observa- 

 tion, that Presnel's theory of diffraction, may not apply in the case of 

 a solar eclipse, and he suggests that at different moments the phases 

 of the light waves may change so that they no longer form a con- 

 tinuous periodic series, and that it is possible, at such great distances, 

 that the interior of the shadow may not be entirely dark, and that 

 sufficient light may come inside to give to- an observer on the earth 

 the appearance of a bright aureole around the moon. f 



Professor Hastings considers the simpler explanation of his obser- 

 vation which has been suggested, that the change in length of the 

 line which he observed might be due to a scattering by our air of the 

 light from the brighter part of the corona, and, therefore, might not 

 indicate any change in the corona itself during the progress of the 

 eclipse, to be untenable, on the ground that the airjwas^too clear, and 

 "diffusion absolutely insensible." He supports this strong state- 

 ment by saying that the photographs taken by the English and by the 

 French observers showed a sensibly black moon, and that " in the 

 photograph of the coronal lines, H and K, taken by the English 

 observers these lines ended abruptly at the moon's edge. "J 



* Eeport B. A. Ad vane. Science, 1872, p. 6. 



f Eeport of Expedition to Caroline Island, 1883, "Washington, p. 105. 

 X Idem, p. 107. 



