458 NEW OBSERVATIONS OF THE PLANET MEECURT. 
case the pliase is obtained by DOting deviation from a straight line joining the 
apparent cusps. We see, then, that in the case of the drawings we have a differ- 
ential catching np in the visibility of the cusps between theoretic and observed 
dichotomy of 2°. This shows again that there is a loss, but does not state its amount 
at either time. 
40. Amount of Loss. — Measures. — For the loss in the case of the measures 
r 
we may proceed as follows. The loss of visibility at the centre of the terminator, 
except when the libratory swing brings dark markings upon it, must be constant. 
Now it appears from Table XL that, after making all corrections for irradiation — 
and inspection will show that no conceivably possible amount of correction will 
further materially affect the result- — there is a difference of 10° between the phase 
loss at and near dichotomy and later on. This then would mark the loss at the cusps, 
other things being equal. The mean taken later on was carefully taken between 
dates when the centre of the terminator w\as as free as possible from the presence 
of dark markings. It must, however, be noticed (see the map) that the terminator 
crossed at the later period a much darker region than was the case at the time of 
dichotomy. 
On the other hand, it is to be remembered that at most observatories the phase 
loss would bo much greater, owing to poorer air. So that we may take provisionally 
10° for the phase loss of the usual observations of Mercury. 
41. Discrepancy in Mercury's Right Ascension accounted for. — Now the loss of 
the cusps and the increased value for the diameter explain a discrepancy in the right 
ascensions of the planet, noticed by Newcomb and published in his Astronomical 
Constants. He 
say 
« The reduction of the semidiameter of the planets was a point to which special 
attention was given. In the case of Mercury, the adopted semidiameter at distance 
unity was 3".34. . . . When the published Right Ascension was that of the centre of 
light simply, a reduction to the true centre was computed by the empirical formula 
used in Washington observations. ... It was assumed that when the illumination 
was such that the thickness of the crescent approached zero, the point observed would 
be two thirds of the way from the centre of the planet to the limb, and that when the 
planet was dichotomized the centre of observation would be five twelfths of the way 
from the centre to the limb. ... The value of S r" therefore indicates that there is a 
remarkable systematic difference in the observed Right Ascension according as the 
planct^ is east or west of the Sun, and therefore according to the illuminated side. 
The sign of the result shows that the reduction to the centre of the planet was 
