S Newcomb on observing the coming Transits of Venus. 75 
of observation, and without a good system of codperation 
among the several parties. For this reason I beg leave to 
call the attention of the Academy to a discussion of the meas- 
ures by which we may hope for an accurate result. 
In planning determinations of the solar parallax from the 
transits of Venus, it has until lately been the custom to depend 
entirely upon observations of the internal Jane: of the limbs 
of the sun and planet, as proposed by Halley. It is a little 
remarkable that while astronomical observations in general have 
attained a degree of accuracy wholly unthought of in the time 
of Halley, this particular observation has never been made with 
a precision at all approaching that which Halley believed that 
he himself had actually attained. In_his paper he states that 
he was sure of the time of the internal contact of Mercury and 
and the sun within a second.* The latest observations of a 
transit of Mercury, made in November, 1868, are, as we shall 
5 sently see, uncertain by. several seconds. It is also well 
own that the observations of the last transit of Venus, that 
of June, 1769, failed to fix the solar parallax with the certainty 
which was looked for, the result of the standard discussion 
being now known to be erroneous by one-thirtieth of its entire 
pon Be 
The discrepancies which have always been found in the class 
of observations referred to, yin the results of pe pie 
itself in this form: When we view a bright £ body ; Peed 
upon a dark ground, the apparent contour of the bright body 
ed beyond its actual contour. It may be generalized as 
llows :—A lucid point, however viewed, presents itself to the 
sense, not as a mathematical point, but as a disc of appreciable 
extent, and, usually, of irregular outline. But, for our gs 
purposes the form of the disc may pe considered circular. Its 
outline is necessarily quite indefinite, and its magnitude in- 
creases oti eee 7 point A bright age being 
composed nity of Luci ints, its apparent enlarge- 
ment is an evident neat ae f this law * re : 
e foll diagrams show the effect of this law u the 
ap 
shows the geometrica form of a aren of the be i there Panini 
Figure 2 shows ‘hele correspon ppearance immediately afte: 
the contact. To indicate —— of irradiation, or +o -lkiw 
the phenomenon as it will actually appear on the theory of irra- 
* Philosophical Transactions, No. 348, p. 454, 
