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succeeded in occupying.* Immediately after the first indenta- 
tion of the sun’s limb had been observed, M. Mouchez began to 
measure the distance between the cusps. “About a quarter of 
an hour,” he proceeds, “ after the first contact, when half the 
planet was still outside the sun, I perceived suddenly the entire 
disc of Venus, defined by a pale halo, brighter near the sun 
than at the summit of the planet ” (that is, at the part remotest 
from the sun’s edge). “ To make sure that I was not under an 
illusion as regards this unexpected phenomenon I immediately 
reversed the position-circle of the micrometer to 180°, and 
measured that diameter of Venus which was still partly outside 
the sun, and I found it identical with the diameter perpendicu- 
lar to the lines joining the centres ; it was, therefore, really the 
entire well-defined disc of the planet which I saw. But, in 
proportion as the second contact approached, the two extreme 
portions of the halo nearest the sun (and more distinctly seen) 
tended to unite, surrounding the segment still exterior to the 
sun with a brighter light, and this too early union ( reunion 
anticipee) of the cusps by a luminous arc was rendered still 
more perfect by a narrow border of very bright light bounding 
the aureole on the disc of Venus. Foreseeing at once that 
there would be great difficulty in observing the geometrical 
contact, even if it would not be absolutely impossible, I quickly 
changed the darkening glass of pale blue for one of deeper tint, 
by means of which I hoped to extinguish this halo with its 
accidental gleams (lueurs accidentelles ), but it was useless: 
the halo still remaining visible, I was obliged to take the 
original darkening glass again. Under such circumstances, I 
had to take as the moment of contact, not the meeting of the 
two cusps, or the geometrical contact, but the moment when 
the sun’s disc no longer seemed disturbed by the bright light 
which enveloped the planet at the point of contact. I observed 
a very sensible time-difference between the moment when I 
believed the contact might have been established and the mo- 
ment when I felt absolutely certain that contact was established. 
.... The third contact was also observed under excellent 
conditions, in very clear sky between clouds, with the same 
phenomena as at the second contact, but in reverse order.” 
It needs no elaborate argument to show that this peculiarity 
must altogether prevent the observation of contacts with the 
degree of accuracy necessary to improve our estimate of the 
sun’s distance. With the improvement of telescopes the phe- 
* At another of these myths (Campbell Island) the French observers had 
bad weather, unfortunately, during the transit. But the Germans, at a 
third myth (Auckland Island), saw the whole transit. 
