1909-10.] Illuminating Power of Groups of Pin-hole Burners. 55 
before. The graphs of w against x increased very rapidly after x reached 
about 2 cm., being nearly the straight line w — 1 before that point. This 
e lx 
suggests the law w= 1H — . The values of l were obtained from the 
mxlO n 
ratios of the successive larger values of w, corresponding to the smaller 
values of P, and then the large constant mxlO” calculated directly, so as 
to reduce w to the values it has on the graphs. Finally, we have the 
formula 
P 2 = 1 + ae~^ 2 / 1 + - — ' _ 1 . 
2 I m x 1(P J 
It will be seen from the tables that the values of the illuminating power 
calculated according to this formula agree with those observed, within the 
error limit. 
The curves in figs. 4 and 5 are the graphs of P 2 . 
