he position of the centre of
Venus with respect to the centre of the sun to be determined by
purely geometric methods. One reason for relying entirely on this
was that the diameter of the sun, as photographed, would be greater
the greater the intensity of the photographic impression, so that
no reliance could be placed upon its uniformity.

Ours were the only parties whose photographic apparatus was fitted
up in this way. The French used a similar system, but without the
essentials of the plumb line and the measurement of the length of
the telescope. The English and Germans used ordinary telescopes
for the purpose.

One of the earliest works of the commission was the preparation
and publication of several papers, which were published under the
general title, "Papers relating to the Transit of Venus in 1874."
The first of these papers was a discussion of our proposed plan of
photographing, in which the difficulties of the problem, and the
best way of surmounting them, were set forth. The next, called
Part II., related to the circumstances of the transit, and was
therefore entirely technical. Part III. related to the corrections
of Hansen's table of the moon, and was published as a paper relating
to the transit of Venus, because these corrections were essential
in determining the longitudes of the stations by observations of
the moon.

In England the preparations were left mostly in the hands of
Professor Airy, Astronomer Royal, and, I believe, Captain Tupman,
who at least took a leading part in the observations and their
subsequent reduction. In France, Germany, and Russia, commissions
were appointed to take charge of the work and plan the observations.

As cooperation among the parties from different countries would be
generally helpful, I accepted an invitation to attend a meeting of the
German commission, to be held at Hanover in August, 1873. Hansen was
president of the commission, while Auwers was its executive officer.
One of my main objects was to point out the impossibility of obtaining
any valuable result by the system of photographing which had been
proposed, but I was informed, in reply, that the preparations had
advanced too far to admit of starting on a new plan and putting it
in operation.

From the beginning of our preparations it began to be a question
of getting from Congress the large appropriations necessary for
sending out the expeditions and fitting them up with instruments.
The sum of