31G CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
the exposure increases, which shows that this detail is due to delicate rather than to 
abrupt changes in intensity. 
I conclude, therefore, that the effect of the light of the sky on photographs taken 
during Sun light, when the atmospheric conditions are at all comparable with those at 
Grenada, would be to entirely alter the form of the corona, in so far as that form is 
made visible by gradual changes of luminosity; and that, with regard to abrupt 
changes of luminosity, they would have to be exceedingly well marked before they 
would be visible in the Sun light photographs, whereas, as a rule, the form of the 
corona is indicated by somewhat delicate shading. 
Thus, the possibility of photographing the corona in Sun light appears to depend on 
the extent to which at high altitudes, or at other localities, more favourable atmo¬ 
spheric conditions may be found. With clearer air, both the Sun and the corona will 
be brighter and the air glare less, but to what extent this change may be hoj^ed for 
I do not know. It is to be observed, however, that besides air glare we have to deal 
with internal reflection, which increases with the increased brilliancy of the Sun. If 
my exposure, short as it was, was in reality sufficient for the internal reflection to 
produce an effect on the plate, with longer exposures and a brighter Sun, but under 
otherwise more favourable conditions, this might become a serious source of trouble. 
The limb of the Moon has undoubtedly been seen for some time after totality (not 
at Grenada, where no doubt the atmospheric conditions were too unfavourable), and, 
if visible, it should, I think, be possible to photograph it. But we must be careful in 
using this as an argument in favour of the possibility of obtaining photographs of the 
corona in Sun light. In the first place, this phenomenon has generally been observed 
within a few minutes of totality. For example, Mr. S. P. Langley, in 1878, at an 
elevation of 14,000 feet, was able to observe the limb of the Moon for 4 minutes 
12 seconds after the reappearance of the Sun without taking any precautions to shield 
his eyes."^ Mr. H. H. Turner has kindly calculated for me that at this time only 
0'068 of the Sun’s surface was exposed; and, as the outer rim of the Sun has only 
about half the brilliancy of the central parts, it may be safely assumed tliat the total 
light emitted was not more than one-twentieth of full Sun light. If the air glare 
varies as the total light emitted, and if, as has been assumed for the purposes of argu¬ 
ment, the ratio of air glare to corona were .50 to 1 in full Sun light, it would only be 
21 to 1 under these circumstances. This would, of course, make the limb of the Moon 
much more readily visible against the corona. This argument does not apply to the 
cases where the limb of the Moon has been seen long before or after totality, but as 
long as the cusps are fairly sharp another circumstance must be considered. In all 
the photographs which I took during the partial eclipse in which there is a false 
corona, it will be seen that it is distributed round the mass of the Sun as a centre, 
and does not uniformly fringe its outline. The cusps often protrude out of it, and 
are quite free from it. Hence, the corona will be less overpowered by the air glare 
* ‘ Nature,’ November 18, 1886. 
