60 D. P. Todd—Solar Parallax from the Velocity of Light. 
If the generally accredited theories of the solar parallax and 
inter-related facts and phenomena are true, the better class of 
these determinations should yield values of the parallax in 
consistent harmony with each other—modified only by deduc- 
tive consideration of the amount of accidental and systematic 
error with which they are severally affected. The well known 
ct, however, is that even the best of these determinations 
appear, at present, singularly and unaccountably discordant. 
he solar parallax, 8’’°848, derived by Professor Newcom 
nearly thirteen years ago, generally replacing Encke’s value, 
8’’'57116, was regarded with caution, only because it was con- 
sidered too small—the researches of Hansen, of Le Verrier, of 
Stone, and of Winnecke were thought to have defined the 
parallax far outside Newcomb’s value. Within two or three 
years, however, the paralluctic pendulum has swung quite to 
the lesser extremity of its arc, until the true value of the solar 
parallax has appeared possibly below 8’°8—and that, too, with 
good reason. But a slight gravitation toward a central value 
is already beginning to show itselfi—and now, in reality, it is 
not at all possible to say that the mean equatorial horizontal 
parallax of the sun is so much as a hundredth part of a second 
different from the ancient figures, 8’°813 [27’2 centesimal], 
adopted by Laplace in the Mécanique Céleste, and given by the 
first discussion of the Transits of Venus in 1761 and 1769. 
The method of determining the solar parallax through the 
velocity of light, though dependent on the results of physical 
experiments conducted under necessarily limited conditions, 
has never given a value of the parallax at all inconsistent wit 
a combination of the best of the purely astronomical deter- 
minations. And this consideration encourages indulgence of 
the hope that, at some time in the not far distant future, this 
method will define the solar parallax within very much smaller 
limits than astronomers have yet known. ‘To show what the 
method is competent to at the present moment is the object 
of this paper. 
bring together all the determinations of the velocity of 
light which have at all the merit of trustworthiness, 
I. Fizeau made the first experimental determination of the 
velocity of light, in 1849; his experiments, however, hardly 
signify more than the completion of the first great step of 
proving the determination to be a physical possibility. The 
first reliable determination was executed thirteen years later by 
Foucault. His work has never been published im extenso, but 
brief papers have appeared in the Comptes Rendus, vol. lv, 
1862, and in Poggendorff’s Annalen, vol. exviii, 1868. The 
resulting velocity of light is 298,000 kilometers per second, in 
which Foucault expresses confidence to about one-six-hun- 
