ON THE TOTAL SOLAN ECLIPSE OF AUGUST 29, 1886. 
313 
sufficient to produce any effect on chloride dry plates with an exposure which is 
sufficient for the air glare or false corona. 
Captain Abney pointed out to me—and I think correctly—that the sky or back¬ 
ground to the Sun, as shown on the photographs, is due to two causes : first, the 
light of the sky, and, secondly, the reflected light from the interior of the instrument. 
Dispersion of light from the mirror or its surroundings would shed a uniform light 
over the whole surface of the exposed plate ; whereas the sky is brightest near the 
Sun, and possibly not always quite uniform or regular in its appearance. Now, on 
several of the photographs that I took there is a perfectly uniform light over the 
whole sky, with no apparent increase of brightness near the limb of the Sun. This 
constitutes the only hopeful sign I have seen ; because, if the above views are correct, 
it indicates that I have not given a sufficiently long exposure to photograph the sky 
round the Sun, and gives hopes that the false sky that I did obtain might be 
diminished by instrumental alterations. 
7. Discussion on the possihility of obtaining Photographs of the Corona during 
Sun Light. 
But there are certain considerations which appear to me to indicate that a practical 
method of photographing the corona during Sun light is not likely to be obtained. 
If I am right in considering that the increased density round the Sun in these 
photographs is a true picture of the sky, and not due to irradiation or internal 
reflection, then it will be seen that this sky illumination is not uniform round the 
Sun, and that the form of the false corona thus formed would always be difficult to 
distinguish from the form of the true corona, and would be liable to be mistaken for 
it. It is quite possible that changes in the appearance of the sky may be more visible 
when photographed than when seen direct, and that these changes may be as sudden 
as that from blue sky to cloud. At high altitudes this difficulty would no doubt be 
lessened, but it would, I think, always be a source of trouble. If, however, these 
irregularities in the false corona are not due to irregularities in the sky, I can only say 
that the instrumental defect which causes them is a very difficult one to cure. 
In considering the utility of the photographs of the corona taken in Sun light, if 
obtainable, the effect of the image being superimposed on the much denser picture of 
the air glare should be carefully considered. In the first place, it is to be observed 
that the air glare or light of the sky increases rapidly as the Sun is approached, and 
that the light of the corona also increases in a similar manner. This will, I think, 
cause the corona to be distinguished with difficulty in the photographs taken in 
Sun light. This may not be so much the case when considering abrupt changes of 
shade in the corona, but, in so far as the form of the corona is distinguished by a 
gradual change of light, it certainly will be. In the following figures, which have no 
numerical significance whatever, and Avhich are merely intended to iliustrate my 
argument, I have assumed the corona to be only from 10 to 15 times less bright than 
MDCCCLXXXIX. —A. 2 S 
