392 Scientific Proceedings, Royal Dublin Society. 
wire, and the moment of last contact with the wire: the mean 
of all three being taken as the time of apparent central transit of 
the spot. 
This time of apparent central transit is affected by three 
distinct sources of error, which cannot be ignored when accurate 
results are sought. These may be described as (1) the correction 
for parallax, (2) the correction for the velocity of light, and (8) the 
correction for phase. 
Taking these in order we have :— 
(1). The correction for parallax. This is due to the change 
in the relative positions of Jupiter and the Harth in the interval 
between two apparent central transits. By the period of rotation 
is of course meant a “sidereal”’ rotation, ¢.e. the interval which 
elapses between two transits of the spot, as it would appear 
S 
Fig. 2. 
from a fixed point at an infinite distance. This is very different 
from the interval between two transits, as seen from a moving body 
like the Karth, comparatively speaking, in the vicinity of the planet. 
