182 Biographical Account of [Marea, 
in this imperfect state of thedata. It is well known that the obser- 
vations which best serve the purpose of determining the orbit of a 
planet are those made at its oppositions to the sun, when an ob- 
server in the earth and in the sun would refer the planet ‘to the 
same point in the starry heavens, or when, in the language of 
astronomers, its heliocentric and geocentric places coincide. Of 
these oppositions in the ease of this planet there were yet only four 
which had been actually observed. Dr. Herschell had, however, 
discovered the planet soon after the opposition of 1781 was passed, 
and though of course that opposition was not seen, yet from the 
observations that were made so soon after, Professor Robison thought 
he could deduce the time with sufficient accuracy. The opposition 
of the winter 1786 he observed himself; for though there was un- 
fortunately no observatory at Edinburgh, he endeavoured to supply 
that defect on the present occasion by a very simple apparatus, viz. 
a telescope on an equatorial stand, which served to compare the 
right ascension and declination of the planet with those of some 
known stars which it happened to be near. His general solution of 
the problem is very deserving of praise; and though the method 
pursued is in its principle the same with all chose which ever since 
the time of Kepler have been employed for finding the elements of 
a planetary orbit, it appears here in a very simple form, the con- 
struction being wholly geometrical, and easily understood. The 
elements, as he found them, are not very different from those that 
have since been determined from more numerous and more accurate 
observations. 
When Dr. Herschell first made known this most distant of the 
planets, many astronomers believed that they had discovered the 
source of those disturbances in our system which had not yet been 
explained. Professor Robison was of this number; for he tells us 
in the beginning of his paper that he had long thought that the 
irregularities in the motion of Jupiter and Saturn, which had not 
been explained by the mutual gravitation of the known planets, 
were to be accounted for by the action of planets of considerable 
magnitude, beyond the orbit of Saturn. Subsequent inquiry, how- 
ever, has not verified this conjecture ; the irregularities of Jupiter 
and Saturn have since been fully explained, and are known to arise 
chiefly from their action on one another, a very small part only 
being owing to that of the Georgium Sidus, or of any of the other 
planets. 
The next publication of Professor Robison was a paper in the 
second volume of, the same Transactions, On the Motion of Light, 
as affected by Refracting and Reflecting Substances, which are 
themselves ia Motion.* 
The phenomena of the aberration of the fixed stars are well 
known to depend on the velocity of the earth’s motion combined 
with the velocity of light; the quantity of the aberration, when all 
* Edinburgh Transactions, vol. ii, p. 83. 
