"As it has been shown that the spherical figure of a cluster is owing to the action of central powers, it follows that those clusters which, cæteris paribus, are the most complete in this figure, must have been the longest exposed to the action of these causes. Thus the maturity of a sidereal system may be judged from the disposition of the component parts.
"Hence planetary nebulæ may be looked on as very [Pg 90]aged. Though we cannot see any individual nebula pass through all its stages of life, we can select particular ones in each peculiar stage." (1789.)
There is something almost grandiose and majestic in his statement of the ultimate destiny of the Galaxy:
"To him the fates were known
Of orbs dim hovering on the skirts of space."
"—Since the stars of the Milky Way are permanently exposed to the action of a power whereby they are irresistibly drawn into groups, we may be certain that from mere clustering stars they will be gradually compressed, through successive stages of accumulation, till they come up to what may be called the ripening period of the globular form, and total insulation; from which it is evident that the Milky Way must be finally broken up and cease to be a stratum of scattered stars.
"The state into which the incessant action of the clustering power has brought it at present, is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is nevertheless certain that since the breaking up of the Milky Way affords a proof that it cannot last forever, it equally bears witness that its past duration cannot be admitted to be infinite." (1814.)
Herschel's relations with his cotemporaries were usually of the most pleasant character, though seldom intimate. This peace was broken but by one unpleasant occurrence. In the Philosophical Transactions for 1792, Schroeter had communicated a series of observations made with one of Herschel's own telescopes on the atmospheres of Venus, the Moon, etc. It was not only an account of phenomena which had been seen; it was accompanied by measures, and the computations based on these led to heights and dimensions for mountains on Venus which were, to say the least, extravagant. The adjective will not seem too strong when we say that the very existence of the mountains themselves is to-day more than doubtful.
The appearances seen by Schroeter were described by him in perfectly good faith, and similar ones have been since recorded. His reasoning upon them was defective, and the measures which he made were practically valueless. This paper, printed in the Transactions of the Royal Society, to which Schroeter had not before contributed, appears to have irritated Herschel.
No doubt there were not wanting members of his own society who hinted that on the Continent, too, there were to be found great observers, and that here, at least, Herschel had been anticipated even in his own field. I have always thought that the memoir of Herschel which appeared in the next volume of the Transactions (1793), Observations on the Planet Venus, was a rejoinder intended far more for the detractors at home than for the astronomer abroad. The review is conceived in a severe spirit. The first idea seems to be to crush an opposition which he feels. The truth is established, but its establishment is hardly the first object.
It seems as if Herschel had almost allowed himself to be forced into a position of arrogance, which his whole life shows was entirely foreign to his nature. All through the review he does not once mention Schroeter's name. He says: