372 Scientific Proceedings, Royal Dublin Society. 
By taking different values of a the following points on this 
curve were found :— 
Sun’s altitude. Theoretical ordinate. Abscissa (mean time). 
H. M. 
48° 45' 50 2 12:6 
45° 53°56 2 46°3 
40° 60°11 3 22°6 
35° 70°05 3 58 
29°) 15) 90 4 37 
25° 116°6 5 «6°4 
This curve is represented in fig. 1 by the dotted curve AKCD, 
and it will be seen that it comes remarkably close to that drawn 
freelyjas the continuation of the branches unaffected by the eclipse 
in the observed curve. 
Another curve representing the decrease in actinic effect with 
the Sun’s altitude could be obtained by exposing the actinometer 
on a day of like atmospheric conditions to the day of the eclipse 
and under similar circumstances as far as possible. 
Curve oF Eciipse HFFEcr. 
Having now found these two curves, viz. the observed curve 
during the eclipse and the theoretical curve of decline of actinic 
power, if a curve be plotted whose abscissee are the mean time and 
ordinates the ratios of the ordinates of the observed curve to the 
corresponding ordinates of the theoretical curve above mentioned, 
this will be the curve representing the eclipse effect alone. For 
equation (2) is 
1 
5 = Q es Qye™ *eosee @. 
while if only a portion s of the area S of the Sun’s dise be exposed 
we have 
1 8 
x =q= 5 Quen Beosee my (6) 
where n is an ordinate of the theoretical curve for the actinic power 
of the Sun at any instant during the eclipse. 
