76 i& Newcomb on observing the coming Transits of Venus. 
diation, we have only to draw an infinity of minute circles, one 
from each point of the sun’s disk visible around the planet. 
8 la. 
The effect of this is shown in the figures la and 2a. The 
exceedingly thin thread shown in figure 1 is thus thickened as 
2. 2a. 
in la, and the sharp cusps of 
figure 2 are rounded off as 
shown in figure 2a. The ap- 
parent radius of the planet 
is diminished by an amount 
= to the radius of the cir- 
of irradiation, and the 
eal of the sun is increased 
y the same amount. om- 
paring figures la and 2a, it 
will be seen that the moment 
of internal contact ‘is marked 
by the apparent formation of 
Ser or “black drop,” between the limbs of the sun and 
the planet. This formation is of so marked a character that it 
