394 Scientific Proceedings, Royal Dublin Society. 
minutes afterits occurrence. Of course, if this delay or retardation 
were constant, it would not affect our observations, but on account 
of our constantly changing distance from the planet this retarda- 
tion will vary between wide limits. Comparing again our obser- 
vations with those made at the moment of opposition, we see that 
the correction to be applied is the difference in the times which 
light requires to travel the distances JH and J, (fig. 2). 
If 7 is the time which light occupies in traversing the mean 
radius of the Harth’s orbit (viz. 8"™°317), this correction is there- 
fore evidently — 
D \ 
(D, - D\P-- (5-1). DE. Ee 
We have, lastly, 
(3). The correction for phase.—Although the great distance of 
Jupiter from the Harth prevents its ever assuming a very marked 
gibbosity, as is seen at each synodical revolution in the case of 
Mars, yet there is a certain amount of phase which will sensibly 
affect observations, such as those with which we are at, present 
concerned. For, the phase is so small—the defect of the diameter 
Fig. 3. 
never exceeding 0’:35—that to the eye the form of the dise is 
always perfectly symmetrical, and consequently the method above 
described of setting the micrometer will have the effect of placing 
the single wire so as to pass through C’, the middle point of 
BD (in fig. 3), instead of through O, the middle point of AB, 
