7 6z THE POPULAR SCIENCE MONTHLY. 



Mr. Woods arrived at the Riffel in the beginning of Jnly, 1884, 

 with an apparatus, similar to one shown in the woodcut on a former 

 page, constructed by Mr. Grubb. 



Captain Abney, who had made observations on the Riffel in former 

 years, had remarked on the splendid blue-black skies which were seen 

 there whenever the lower air was free from clouds or fog. But un- 

 fortunately during the last year or so a veil of finely divided matter 

 of some sort has been put about the earth, of which we have heard so 

 much in the accounts from all parts of the earth of gorgeous sunsets 

 and after-glows. This fine matter was so persistently present in the 

 higher regions of the atmosphere during last summer, that Mr. "Woods 

 did not get once a really clear sky. On the contrary, whenever visible 

 cloud was absent, then instead of a blue-black sky there came into 

 view a luminous haze, forming a great aureole about the sun, of a faint 

 red color, which passed into bluish white near the sun. Mr. Woods 

 found the diameter of the aureole to measure about 44. This appear- 

 ance about the sun has been seen all over the world during last sum- 

 mer, but with greatest distinctness at places of high elevation. 



The relative position of the colors, blue inside and red outside, 

 shows that the aureole is a diffraction phenomenon due to minute 

 particles of matter of some kind. Mr. Ellery, Captain Abney, and 

 some others, consider the matter to be water in the form probably of 

 minute ice-spicules ; others consider it to consist of particles of vol- 

 canic dust projected into the air during the eruption at Krakatoa ; 

 but whatever it is, and whencesoever it came, it is most certainly mat- 

 ter in the wrong place so far as astronomical observations are con- 

 cerned, and in a peculiar degree for success in photographing the 

 corona. We are only beginning to learn that, whether in our persons 

 or in our works, it is by minimized matter chiefly that we are undone. 

 So injurious was the effect of this aureole that it was not possible to 

 obtain any photographs of the corona at my observatory near London. 

 This great diffraction aureole went far to defeat the object for which 

 Mr. Woods had gone to the Riffel, but fortunately the great advan- 

 tage of being free from the effects of the lower eight thousand feet of 

 denser air told so strongly that, notwithstanding the ever-present au- 

 reole, Mr. Woods was able to obtain a number of plates on which the 

 corona shows itself with more or less distinctness. [Three untouched 

 photographic copies of the plates taken at the Riffel were shown upon 

 the screen.] From the presence of the aureole the negatives show less 

 detail than we have every reason to believe would have been the case 

 if the sky had been as blue and clear as in some former years. This 

 circumstance makes great care necessary in the discussion of these 

 plates, and it would be premature to say what information is to be ob- 

 tained from them. 



[As an illustration of the differences of form which the corona has 

 assumed at different eclipses, photographs taken in 1871, 1878, 1882, 



