ON THE TOTAL SOLAR ECLIPSE OF AUGUST 20, 1880. 
317 
near the cusps than at other parts of the limb of the San. In tlie second place, in 
the case of the Moon being seen against the corona, we are dealing with a sudden 
change of shade, and not a gradual one, as in the case of the corona. This, as I have, 
I think, proved, would make it more easily photographed. It would have been 
encouraging to have obtained photographs of the limb of the Moon during the partial 
eclipse; but, unless they were obtained when a large part of the Sun was exposed, it 
would have done little towards proving the possibility of getting photographs of the 
corona in full Sun light. 
It is to be observed that many of these remarks indicating the improbability of 
obtaining results by this method do not hold good in the case of exceptionally well 
marked features of the corona, where the changes of shade are really abrupt, and 
especially if they are radial in their direction. Thus, the first successes that we 
should expect to obtain would be in such cases as that of the “ remarkably formed 
rift” in the corona of 1883, to which such a distinct likeness is reported to have been 
seen in photographs taken in Sun light by Dr. Huggins in England. 
As for hoping to obtain any photographic records in Sun light of the outer limits of 
the corona as visible to the eye during totality, it is, I think, out of the question, as 
the following considerations will show, although the actual numbers quoted may be 
considerably in error. Judging from my own results, 10 seconds with a bromide 
plate would appear to be too small an exposure for the extreme limits. Assuming 
that the exposure I gave during Sun light was one-fifth of a second, and that the 
chloride plates are 30 times less sensitive than the bromide plates, then it would take 
1500 times the exposure I gave during Sun light to photograph the outer parts of the 
corona during totality. On this point it is instructive to compare Mr. Wesley’s 
drawing from the jfirotographs with Captain Archer’s direct drawing, probably one 
of the most accurate ever taken.'^' 
8. Should the Experiment he Tried Again f 
In conclusion, the questions naturally arise : Is the experiment worth trying again, 
and, if so, under what conditions ? As to the latter point, experience seems to show 
that:— 
(1.) It should be tried at a station at great elevation above the sea. 
(2.) It should be tried when the Sun is nearly vertical, during an eclipse of the 
corona by the Moon, Mercury, or Venus. 
(3.) The exposure must be more varied and longer. At present I see no reason 
why increase of exposure should not simply increase the density of the air glare in the 
same proportion as that of the corona, thus gaining no advantage by the change. 
Unless the air glare and internal reflection can be so diminished as to make an 
exposure of one second with an instrument like that used by me a possibility, I fear 
* See Appendix to Captain Abney and Pi’ofessor Thorpe’s paper on tlie Photometric Intensity of the 
Coronal Light during this eclipse (infra, p. 382). 
