INTENSITY OF CORONAL LIGHT DURING THE SOLAR ECLIPSE OP 188G. SOS 
Probably the brightest part of the corona was about 15 times brighter than the 
surface of the full Moon, or 37,000 times fainter than the surface of the Sun. 
It would further seem that the corona of December 22, 1870, was 7^ times brighter 
than that of July 29, 1878. 
Description of Methods Adopted ditring the Eclipse of August 28-29, 1886. 
The instruments used by us for the measurement of the coronal light on this 
occasion were three in number. The first was constructed to measure the comparative 
brightness of the corona at different distances from the Moon’s limb. The second was 
designed to measure the total brightness of the corona, excliuling as far as possible 
the sky effect. The third was intended to measure the brightness of the corona, 
together with the brightness of the sky in the direction of the eclipsed Sun. 
In a paper by one of us, in conjunction with General Festing,* it was shown that 
light of any colour can be measured for luminosity in terms of light of any other 
colour, provided always that the last-named light can be rapidly altered in intensity, 
so that at one time it is evidently below the intensity of the light to be compared, 
and immediately afterwards that it is evidently above it. The oscillations of the 
intensity, if then gradually diminished, finally give the value of the coloured light in 
terms of the luminosity of the light of which the intensity is rapidly changed. 
In a more recent papert it has been shown that the light of a glow lamp may be 
used for measuring the intensity of any other light by making a rapid change in the 
resistance of the circuit. In the photometric measures which are now to be described 
this plan was adopted for ascertaining the value of the coronal light in terms of a 
Siemens unit. Before continuing the description it may be well to note that the 
Siemens unit is very nearly 0’8 of a standard candle. This unit has the advan¬ 
tages that the area of the burner is fixed; that the flame used in the photometric 
measures can always be made of exactly the same height; that the thickness and 
shape of the flame are practically invariable; that the material producing the flame 
can be obtained in commerce ; and that any slight impurity in it has no practical 
effect on the value of the light emitted. Neither the effect of the temperature at the 
time of trial nor the variation due to difference in barometric pressure has been 
thoroughly tested, but there are presumably but slight differences due to these causes. 
At all events, there is nothing to prevent its employment for the object we had in 
view. One experiment may, however, be quoted as regards the luminosity of the 
flame when the temperature was varied some 20°. The lamp was carefully adjusted 
so that the tip of the flame just touched the gauge supplied with the instrument, and 
its value taken against the glow lamp, which was kept at a bright yellow heat by a 
current passing through it. A large photograph of the flame was also taken. The 
* ‘ Phil. Trans.,’ 1886, “ Colour Photometry,” Abnet and Festing. 
t ‘ Roy. Soc. Proc.,’ vol. 43, 1887, Abnet and Resting. 
