116 



Dr. W. Huggins. 



finely- divided matter of some sort in trie higher regions of the air. 

 Mr. Woods observed from the Riff el that when no visible cloud or 

 mist was present, there came into view a great anreole around the snn, 

 about 44° in diameter, of a faint red colour at the outside and passing 

 into bluish-white near the sun. This was clearly a diffraction pheno- 

 menon showing the presence of minute particles of matter of some 

 sort in the higher regions of our atmosphere. 



The abnormally large amount of air-glare from this cause — even on 

 the finest days — prevented any success in photographing the corona in 

 this country, and went far to counteract the advantages of being 

 above the denser strata of air which Mr. Woods would have gained on 

 the Eiffel under ordinary circumstances. 



Mr. Woods sums up his results in the following words : — " Results 

 on the same day are almost, if not quite, alike both with the disk and 

 without. The corona varies more or less from day to day. The 

 clearer the day the better the results."* 



During the last two years the sky in this country has been too 

 bright from scattered light to make it possible to obtain successful 

 photographs of the corona. f 



We have now to discuss the probable nature of the corona. J 



The drawings, but especially the photographs, of the solar eclipses 

 of the last twenty-five years show that notwithstanding great changes 

 in form and in brightness, the corona is permanent in its more funda- 

 mental characters. The observed changes in form, in brightness, and 

 in relative extension, are obviously due to secondary modifications of 

 the conditions to which the corona owes its existence. 



The circular form which was ascribed to the corona in the older 

 observations can scarcely be regarded — even in the roughest sense of 

 the word — as correct. On the contrary, the apparent form of the corona 

 is always very irregular, in consequence of the greater extension and the 

 greater relative brightness of certain parts. Upon the whole, there is 



* "The Observatory," December, 1884, p. 378. 



f It may not be unnecessary to state that what the photographer has to seize 

 upon on his plates is the small excess of photographic power of the air-glare in- 

 creased by the coronal light from behind over that of the air-glare alone. For this 

 purpose the greatest care is necessary to select the most suitable time of exposure, 

 and to arrest tbe slow development of the plate at the proper moment. Unless the 

 attempt is made at a high elevation, the impression upon the plate must be a very 

 slight one, and the developed image can only be seen under favourable conditions of 

 illumination. Grreat care must be taken that all instrumental effects have been 

 carefully eliminated. A convenient way of distinguishing effects upon the plate 

 which are due to the instrument, is to take pictures with the instrument alternately 

 on the west and on the east side of the meridian. 



X The principal points of the discussion of the nature of the corona which follows 

 were suggested in a discourse given at the Eoyal Institution, February 22, 1885, 

 entitled " On the Solar Corona." 



