In the spring of 1808 he was quite seriously ill; but in May the observing went on again. In 1809 and 1810 his principal investigations were upon physical subjects (NEWTON'S rings), and in 1811 the only long series of observations was upon the comet of that year. After 1811 the state of HERSCHEL'S health required that his observations should be much less frequent. Much of the time after 1811 he was absent, and his work at home consisted largely in arranging the results of his previous labors, and in computations connected with them. All through the years 1814 to 1822, HERSCHEL'S health was very feeble. The severe winter of 1813-14 had told materially upon him. In 1814, however, he undertook to repolish the forty-foot mirror, but was obliged to give it over.
He now found it necessary to make frequent little excursions for change of air and scene. His faithful sister remained at home, bringing order into the masses of manuscript, and copying the papers for the Royal Society.
She was sick at heart, fearing that each time she saw her brother it would be the last. In 1818 she says:
"Feb. 11, I went to my brother and remained with him till the 23d. We spent our time, though not in idleness, in sorrow and sadness. He is not only unwell, but low in spirits."
In 1818 (December 16), HERSCHEL went to London to have his portrait painted by ARTAUD. While he was in London his will was made.[28]
In 1819 there is a glimmer of the old-time light. In a note HERSCHEL says:
"LINA:--There is a great comet. I want you to assist me. Come to dine and spend the day here. If you can come soon after one o'clock, we shall have time to prepare maps and telescopes. I saw its situation last night. It has a long tail.
This note has been carefully kept by his sister, and on it she has written: "I keep this as a relic. Every line _now_ traced by the hand of my dear brother becomes a treasure to me."
So the next three years passed away. Sir WILLIAM[29] was daily more and more feeble. He spent his time in putting his works in order, but could devote only a few moments each day to this. His sister says:
"_Aug. 11th_, _12th_, _13th_, and _14th_ [1822], I went as usual to spend some hours of the forenoon with my brother.