S. Newcomb on observing the coming Transits of Venus. 77 
has been generally supposed there could be little doubt of the 
moment of its occurrence. The remarks of the observers have 
given color to this supposition, a black drop se generally 
described as appearing suddenly at a definite m 
Exa figure 2a, it will ‘aa seen that se tien still ap- 
= entirely within the disk of the sun. The geometrica. 
e which bounds the latter, and that which bounds the 
Slants instead of touching, nv separated by an amount equal 
to double the irradiation. the observer waits until the 
now realy widened ae flattened, extending on both sides this 
int. The estimate of the moment of contact must therefore 
died po ough, the ele of estimating being far less accurate 
than those afforded by a common filar micrometer. In the ac 
tual case the eye has to continue the two circles to the point of 
contact by estimation through a distance depending on the 
cope of emmeee while measures with a micrometer are 
vy contact of a wire with a disk. Such estimates 
jaye therefore, been Bacetianity sae - inv: tors, not 
only ‘from their necessary inacc use the time o 
“apparent contact” depends apon the macens of irradiatio: = 
which varies with the observer and the telescope. If there 
no irradiation at all, the time of apparent contact and that of 
true contact will be the same, as shown in a 2; while, 
when the cusps are enlarged by irradiation, apparent ‘contact 
will not occur until the —— has moved throug a space equal 
to double the irradiatio 
ian us _—- to es phenomena at actual contact. Accord- 
the theory as it has been presented, the formation or 
peo of the black ligament connecting the dark body of ¥s 
nus with the dark ground of the sky is a ee a 958 
enon, occurring at the moment of true 
was, I peas — yo eived theory u until Wolf meg ree made 
nus and the dark sky broke off by ea indefinitely thin, 
The result is not ult to account for. Irradiation has al- 
ready been described as a spreading of the light emitted from 
