INTENSITY OF CORONAL LIGHT DURING THE SOLAR ECLIPSE OF 1886. 381 
In comparing the observations of Lieutenant Douglas and Lieutenant Bairns- 
FATHER with those made during the 1878 Eclipse it must be remembered that the 
conditions of observation on the two occasions were widely different. The observa¬ 
tions in the West Indies were made at the sea-level, in a perfectly humid atmosphere, 
and with the Sun at no greater altitude than 19°. Professor J. W. Langley observed 
O 
from the summit of Pike’s Peak, which is 14,100 feet above the sea, in a relatively dry 
atmosphere, and with the Sun at an altitude of 39°. Dr. J. C. Smith’s position at 
Virginia City, Montana, was about 6,000 feet above the sea-level, and the Sun’s altitude 
at the time of observation was about 44°. If we have regard then to the extinction 
of the coronal light in the Earth’s atmosphere, it follows, ccEteris ijarihus, that the 
observations during the 1886 Eclipse should show a much lower photometric intensity 
than those of 1878, even if the intrinsic brightness of the corona was the same on the 
two occasions. 
It will be a matter of remark that the brightness of the corona at the various points 
measured is very inferior to that of the Moon’s surface. The value of the light from 
the full Moon has been variously estimated, but it is not an unfair estimate to take it 
as '02 of a candle at 1 foot distance. Supposing the whole surface of the Moon to be 
of equal brightness, the brightness of the Moon’s image on the photometric screen used 
. in the equatorial telescope compared with moonlight itself would have been very 
closely 60 X ’02 candle, or 1'2 candles. No matter what part of the image fell on 
the grease spots, it would have required this illumination to have made the grease 
spots disappear. If we take the highest reading of the corona measured, we find that 
it is ’07 of a Siemens unit, or about ‘06 caudle. It thus appears that the brightness 
of the brightest measured part of the corona (1’55 Solar semi-diameters) was 200 times 
less bright than that of the surface of the Moon, whilst the furthest spot at 3'66 Solar 
semi-diameters, having a value of ’019 Siemens unit, or ‘015 candle, was only of 
the brightness. 
The highest value of the coronal light, measured in the ordinary photometer, was 
about '02, or equivalent to' that of the full Moon. It is evident therefore that, even 
when taking into account the greater angular area wdiich the corona occupied, the 
brightness of the corona w^as very much greater close to the limb than elsewhere. 
The photographs which were taken show this fact. A dense image of the Moon 
might be secured on plates such as were used at the eclipse in the of ^ second. 
The photographs which were taken of the corona varied in exposure from half a second 
to 100 seconds. Even with the first named exposure the corona close to the limb was 
much over exposed, showing the intense brightness of that part, whilst the image of 
the highest part of the corona measured was hardly visible. Owing to the variable 
quantities of cloud during totality, it would be unfair to try to make any comparison 
between the brightness of the corona at the poles and at the equator, though, had no 
cloud intervened, the arrangements adopted would have enabled such to be done. 
Nor is it worth while to endeavour to establish any law for the decrease of brightness 
