74 Dr. Herschel's Discovery of four additional 
will obstruct an inferior one; but some circumstances which 
attend the operation of the affections of light upon the eye, 
when objects are very faint, are so remarkable, that they must 
not be passed over in silence. 
After having been used to follow up the satellites of Saturn 
and Jupiter, to the very margin of their planets, so as even to 
measure the apparent diameter of one of Jupiter's satellites by 
its entrance on the disk,* I was in hopes that a similar oppor- 
tunity would soon have offered with the Georgian satellites : 
not indeed to measure the satellites, but to measure the planet 
itself, by means of the passage of the satellite over its disk. 
I expected also to have settled the epochs of the satellites, 
from their conjunctions and oppositions, with more accuracy 
than I have yet been able to do, from their various positions 
in other parts of their orbits. A disappointment of obtaining 
these capital advantages deserves to have its cause investigated ; 
but, first of all, let us cast a look upon the observations. 
The satellites, we may remark, become regularly invisible, 
when, after their elongation, they arrive to certain distances 
from the planet. In order to find what these distances are, we 
will take the first observation of this kind, as an example. 
Feb. 22, 1791, the first satellite could not be seen. Now, by 
my lately constructed tables, its longitude from the apogee, at 
the time of observation, was 204,5 degrees ; that is, 24,5 de- 
grees from the most contracted part of its orbit, on the side that 
is turned to us, which, as its opposite is called the apogee, I 
shall call the perigee. By my tables also for the same day, we 
have the distance of the apogee from the planet, which is ,6 o ; 
supposing the greatest elongation distance to be 1. This 
* See Phil. Trans, for 1797, Part II. page 335. 
