4G0 NEW OBSERVATIONS OF THE PLANET MERCURY. 
43. Effect of Testudo regio. — To begin with, we must note that the onlj 
extrinsic or predicable cause for variation in the differences from top to bottom is to 
be found in the notes at the side. At the times there specified the long dark 
o 
called Testudo regio was in the act of passin 
terminator 
quence of the libration. And this happened presumably a little earlier for the 
measures than it did for the drawings, first, because tlie eye can see better when 
not hampered by wires near the object observed, and can therefore in the case 
of the terminator see a little farther out into the decreasing Illumination; and 
secondly, because in taking the measures a somewhat higher power was used, 167 
as against 140. 
This is the only cause for variation which we can eliminate from the discussion 
by taking account of it at the start. 
44. Increase of Visibility of the Cusps. — Considering now the respective col- 
umns of the differences of the drawings and the measures as compared with the 
theoretic phase, we see that the differ 
being, both sets of them, small 
and near dichotomy, then in increasing, and then in decreasing slightly 
conjunction. The increase, which for the measures "culminates about March 3 
part due to the passing of Testud 
Eliminating its effect 
we find a fairly steady increase starting from the time when the phase was greatest up 
to the time that foreshortening began to work, on approaching superior conjunction. 
The general cause underlying this was the increase of visibility in the cusps of the 
polar diameters already considered in Article 33. How definite a factor it is, is seen 
more clearly yet by continuing on to an even greater phase, where from its nature 
it became more marked. We cannot do this with the measures because of having 
none, but fortunately we can do it with the measures of the drawings ; and this 
is shown in Table X. 
45. Foreshortening due to Irradiation. — Lastly, we perceive what we should 
expect to find, a falling off of the differences as we approach conjunction, owing to 
the mcrease of irradiation at the terminator due to foreshortening, an apparent 
which 
polar diameter is not able to overcome 
In the following ratios the polar diameter is also corrected for irradiation, fi 
correction amounting to twice 0M3, or to 0^26, at conjunction, making it 7^54. 
The value 7".50 is used throuo^hout 
