ON THE TOTAL SOLAR ECLIPSE OF AUGUST 29, 188G. 
329 
A faint Fraunhofer line lias just been mentioned as apjiearing in the corona 
spectrum. The appearance of dark lines has been a source of considerable trouble in the 
reduction of the photographs. After all the measurements had been made, it struck 
me that some lines which I had put down as corona lines were really only the 
intervals between Fraunhofer lines, and 1 had to subject the photograph to a further 
careful examination. The effects produced by the overlapping of a spectrum of dark 
lines over one of bright lines is very complicated ; especially, apparently, some of the 
weaker Fraunhofer lines can be traced, while some of the stronger ones do not make 
their appearance. I believe that this is due partly to the overlapping of bright and 
dark lines, but principally to an optical effect of contrast. 
A bright line shows black on the negative and is bounded on both sides by an 
apparently lighter background. This is a welhknown contrast effect. The H and K 
lines, for instance, seem to be surrounded by a lighter band, which follows the contour 
not only of the lines, but also of the wing by the side of the prominence. If, now, a 
Fraunhofer line happens to be by the side of a bright line, the contrast is strengthened, 
and both the bright and the dark lines appear more distinctly than they otherwise 
would. This is the only simple way in which I can explain some of the appearances 
of the photographs. 
The triplet with a wave-length 4026'0, 4029’7, and 4036 8, which is represented in 
fig. 1, Plate 9, is a case in point; the group of Fraunhofer lines at 4031, about, is 
weaker than the strong Fraunhofer line 4045, and yet is much more easily visible on 
the eclipse photographs. This I believe to be due simply to the fact that the two 
lines 4029'7 and 4036‘8 set off by contrast the intervening group of Fraunhofer lines. 
Such contrast effects may, in some cases, have materially affected my measurements. 
I have made these as carefully as I could, and I believe that the great majority of the 
lines I have put down as corona lines are really such, but some of the weaker lines 
may be due to the optical efilects I have just described. 
Possibly the true spectrum of the corona may be still further complicated owing to 
the following cause :—The base of the corona gives us evddently a strong continuous 
spectrum, and it is possible that the lines of the outer corona may therefore appear as 
dark lines in a bright background. Captain Abney found some Fraunhofer lines 
reversed over the face of the Moon, while the G band was absent. The cause I have 
suggested may account for this. All these considerations show how very careful we 
must be in the interpretation of photographs of the coronal spectrum. 
I had intended to have made a careful drawing of all I can see on the photographs. 
Figs. 2 and 3, Plate 9, are specimens of certain portions of the spectrum on a scale 
40 times the original.'" I had, however, to give up the work, as I found it too trying 
to the eyes. The length of spectrum represented in fig. 3 is in the original about 2 mm. 
* The distances between the horizontal bands are magnified on a slightly different scale in the tw’o 
drawings, so as to make them equal, while as before explained, the real distances decrease towards the 
violet. 
MDCCCLXXXIX.-A. 
2 u 
