ts of the kind were thrown into the
shade by Professor Newcomb's arduous operations at Washington in
1880-1882.[791] The scale upon which they were conducted was in itself
impressive. Foucault's entire apparatus in 1862 had been enclosed in a
single room; Newcomb's revolving and fixed mirrors, between which the
rays of light were to run their timed course, were set up on opposite
shores of the Potomac, at a distance of nearly four kilometres. This
advantage was turned to the utmost account by ingenuity and skill in
contrivance and execution; and the deduced velocity of 299,860
kilometres = 186,328 miles a second, had an estimated error (30
kilometres) only one-tenth that ascribed by Cornu to his own result in
1874.

Just as these experiments were concluded in 1882, M. Magnus Nyren, of
St. Petersburg, published an elaborate investigation of the small
annular displacements of the stars due to the successive transmission of
light, involving an increase of Struve's "constant of aberration" from
20.445" to 20.492". And from the new value, combined with Newcomb's
light-velocity, was derived a valuable approximation to the sun's
distance, concluded at 92,905,021 miles (parallax = 8.794"). Yet it is
not quite certain that Nyren's correction was an improvement. A
differential method of determining the amount of aberration, struck out
by M. Loewy of Paris,[792] avoids most of the objections to the absolute
method previously in vogue; and the upshot of its application in 1891
was to show that Struve's constant might better be retained than
altered, Loewy's of 20.447" varying from it only to an insignificant
extent. Professor Hall had, moreover, deduced nearly the same value
(20.454") from the Washington observations since 1862, of Alpha
Lyrae (Vega); whence, in conjunction with Newcomb's rate of light
transmission, he arrived at a solar parallax of 8.81".[793] Inverting
the process, Sir David Gill in 1897 derived the constant from the
parallax. If the earth's orbit have a mean radius, as found by him, of
92,874,000 miles, then, he calculated, the aberration of
light--Newcomb's measures of its velocity being supposed exact--amounts
to 20.467". This figure can need very slight correction.

Professor Harkness surveyed in 1891,[794] from an eclectic point of
view, the general situation as regarded the sun's parallax. Convinced
that no single method deserved an exclusive preference, he reached a
plausible result through the combinati