78 iS Newcomb on observing the coming Transits of Venus. 
each point of the surface viewed, so that every such point ap- 
oe asasmall circle. The obvious effect of this spreading is a 
as to be visible, or so thin as to be invisible. 
irradiation, and the worse the definition, the thicker will be the 
An interesting observation illustrative of this pare was made 
defined phenomenon which can.see is the meeting of t 
bright cusps and the consequent formation of the thread of light 
at ingress and the rupture of the thread at egress. 
To recapitulate our conclusions— 
1. The moment of observed internal contact at ingress is that 
at which the thread of light between Venus and the sun be 
comes thick enough to be visible. 
, least visible thickness varies with the observer 
awe the instrument, and, perhaps, with the state of the atmos 
phere. 
_ 8. The apparent initial thickness of the thread varies with the 
) e. 
Two ques are now to bediscussed. The observed times 
varying with the observer and the instrument, we must know 
how wide the variation may be. If it be wide enough to ren- 
* Astronomische Nachrichten, B. 73, & 209. 
