D. P. Todd—Solar Parallax from the Velocity of Light. 59 
Having filled the lines with graphite in order to obtain a 
sharper definition of the edges, these two plates were then placed 
face to face in a mounting of stiff balsam in the same direction 
as that in which they were ruled. When the lines at one end 
were made coincident, not only were the other end lines coin- 
cident, but every one of the 4001 lines exactly overlapped its 
fellow. With a power of 300 I was unable to detect a single 
case of deviation from exact superposition. 
This experiment merely showed that the machine would rule 
two plates exactly alike under the same conditions, but it gave 
no indications with reference to the homogeneous character of 
the graduations. But when the plates were placed face to face 
in a direction opposite to that in which one of them was ruled, 
then, coincidence being made between the lines at one end, a 
test of the homogeneity was found in the coincidence of the 
remaining graduations. 
This coincidence seemed to be perfect for about two inches. 
At that point there was an abrupt separation amounting to 
to diminish, and finally disappeared at about one inch and one- 
half from the point where it first appeared. 
This error is probably due to a change of temperature during 
hich the machine remained at rest. It is rather 
first instance the overlapping of the edges was not sufficient to 
attract attention. 
Harvard College Observatory, October 28, 1879. 
Art. X.—Solar Parallax from the Velocity of Light; by D. P. 
opp, M.A., Assistant Nautical Almanac. 
‘THE opposition of Mars in 1862, and the experimental deter- 
mination of the velocity of light by Foucault in the same year, 
mark the beginning of a new era in the history of the deter- 
Iination of the solar parallax. Especial prominence has 
attached to the subject ever since, not only from its inherent 
Importance, but also from the rapidly multiplying determina- 
tons of this constant which have been made, and the vigor of 
discussion that has been everywhere prevalent in scientific 
astronomical circles. 
