pman
deduced 8.81",[770] implying a distance 700,000 miles greater than Stone
had obtained. The best French observations of contacts gave a parallax
of about 8.88"; French micrometric measures the obviously exaggerated
one of 9.05".[771]

Photography, as practised by most of the European parties, was a total
failure. Utterly discrepant values of the microscopic displacements
designed to serve as sounding lines for the solar system, issued from
attempts to measure even the most promising pictures. "You might as well
try to measure the zodiacal light," it was remarked to Sir George Airy.
Those taken on the American plan of using telescopes of so great focal
length as to afford, without further enlargement, an image of the
requisite size, gave notably better results. From an elaborate
comparison of those dating from Vladivostock, Nagasaki, and Pekin, with
others from Kerguelen and Chatham Islands, Professor D. P. Todd, of
Amherst College, deduced a solar distance of about ninety-two million
miles (parallax 8.883" +-0.034"),[772] and the value was much favoured by
concurrent evidence.

On the whole, estimates of the great spatial unit cannot be said to have
gained any security from the combined effort of 1874. A few months
before the transit, Mr. Proctor considered that the uncertainty then
amounted to 1,448,000 miles;[773] five years after the transit,
Professor Harkness judged it to be still 1,575,950 miles;[774] yet it
had been hoped that it would have been brought down to 100,000. As
regards the end for which it had been undertaken, the grand campaign had
come to nothing. Nevertheless, no sign of discouragement was apparent.
There was a change of view, but no relaxation of purpose. The problem,
it was seen, could be solved by no single heroic effort, but by the
patient approximation of gradual improvements. Astronomers, accordingly,
looked round for fresh means or more refined expedients for applying
those already known. A new phase of exertion was entered upon.

On September 5, 1877, Mars came into opposition near the part of his
orbit which lies nearest to that of the earth, and Dr. Gill (now Sir
David) took advantage of the circumstance to appeal once more to him for
a decision on the _quaestio vexata_ of the sun's distance. He chose, as
the scene of his labours, the Island of Ascension, and for their plan a
method recommended by Airy in 1857,[775] but never before fairly tried.
This is known as the "diurnal metho