312 
CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
distinguished from the light being diminished by an impure or dense atmosphere. 
Experiments comparing the light of the corona with Sun light immediately before and 
after the eclipse would be necessary to settle this point. 
(4.) The exposure was too short. 1 feel confident that at Grenada an alteration in 
this respect would not have materially altered the result; but, even with the atmo¬ 
sphere in its unfavourable condition, I think a larger exposure should have been 
adopted. Before leaving England I had come to the conclusion that the right length 
of exposure would be that sufficient, and not much more than sufficient, for the air 
glare to produce an effect on the plate. I thought that the corona would be more 
readily visible if the background of the sky was faintly developed on the negative ; 
and in this opinion I think Dr. Huggins would have agreed, as his photographs seem 
to have been treated in that way. I found that my instantaneous shutter, when set 
at its slowest rate, gave the required result, and I was at the time quite contented. 
I have, however, very carefully reconsidered this subject, and now come to the 
conclusion that this was a mistake. The question to be settled is—At what photo¬ 
graphic density do we get the greatest ratio of small changes of shade in the negative, 
as seen by the eye, compared with the changes in luminosity of the objects photo¬ 
graphed ? Captain Abney gives some results in the form of curves in his Text-book 
of Science on Photography, which, if modified to suit this problem, would appear to 
indicate that the ratio is greatest when the photographic density is about one-third 
or half way between white and the deepest shade due to the fall development of the 
image of a bright object.* 
If this is the right interpretation—and I believe it to be so—then it is evident, in 
order to have the best chance of photographing the small difference of shade between 
the sky and the corona, I should have given an exposure which would have allowed 
me to bring up the sky to what I may call a third or a half full density. In order to 
have done this I should have lengthened my exposure. 
Acting in accordance with the advice of Dr. Huggins, I adopted a slow develop¬ 
ment wfith weak solutions. I could not have followed a higher authority, but I must 
confess that I have some doubts whether a longer exposure with a quicker but well- 
restrained development might not have produced better results. The best time for 
watching for faint outlines is during development, and with a well-exposed plate we 
have an opportunity of observing the image in every stage. 
All these considerations appear to me to point to longer and more varied exposures 
than I gave, together with a well-varied development. 
My conclusions are, therefore :— 
(1.) That my results do not prove the impossibility of photographing the corona in 
Sun light. 
(2.) But they prove that under certain circumstances the light of the corona is not 
* See p. 256. The ordinates of his enrves represent the shade as measured by the proportion of black 
to white; the scale of shade as measured by the eye is somewhat different from this. The absciss® 
represent the length of exposure, which I imagine to be eqnixalent to differences of luminosity. 
