This eclipse was also seen by Sir John Herschel at the Cape of Good Hope, as well as by many astronomers in Europe provided with instruments of great optical power. In 1782 Sir William Herschel saw one of the stars of Zeta Herculis eclipse the other.
In the "Connexion of the Physical Sciences" I have given an abridged account of Sir John Herschel's most remarkable discoveries in the southern hemisphere; but I may mention here that he determined the position and made accurate drawings of all the nebulæ that were distinctly visible in his 20 ft. telescope. The work he published will be a standard for ascertaining the changes that may take place in these mysterious objects for ages to come. Sir William Herschel had determined the places of 2,500 nebulæ in the northern hemisphere; they were examined by his son, and drawings made of some of the most remarkable, but when these nebulæ were viewed through Lord Rosse's telescope, they presented a very different appearance, showing that the apparent form of the nebulæ depends upon the space-penetrating power of the telescope, a circumstance of vital importance in observing the changes which time may produce on these wonderful objects.
Long afterwards Lord Rosse wrote in reply to some questions which my mother had addressed to him on this subject:—
FROM THE EARL OF ROSSE TO MRS. SOMERVILLE.
Castle, Parsonstown, June 12th, 1844.
I have very reluctantly postponed so long replying to your inquiries respecting the telescope, but there were some points upon which I was anxious to be enabled to speak more precisely. The instrument we are now using is 3 feet aperture, and 27 feet focus, and in the greater proportion of the nebulæ which have been observed with it some new details have been brought out. Perhaps the most interesting general result is that, as far as we have gone, increasing optical power has enlarged the list of clusters, by diminishing that of the nebulæ properly so-called. Such has always been the case since the nebulæ have been observed with telescopes, and although it would be unsafe to draw the inference, it is impossible not to feel some expectation that with sufficient optical power the nebulæ would all be reduced into clusters. Perhaps the two of the most remarkable of the resolved nebulæ are Fig. 26 and Fig. 55. In several of the planetary nebulæ we have discovered a star or bright point in the centre, and a filamentous edge, which is just the appearance which a cluster with a highly condensed centre would present in a small instrument. For instance, Figs. 47 and 32. We have also found that many of the nebulæ have not a symmetrical form, as they appear to have in inferior instruments; for instance, Fig. 81 is a cluster with long resolvable filaments from its southern extremity, and Fig. 85 is an oblong cluster with a bright centre. Fig. 45 is an annular nebula, like Herschel's drawing of the annular nebula in Lyra. I have sent drawings of a few of these objects to the Royal Society, they were forwarded a few days ago. We have upon the whole as yet observed but little with the telescope of 3 feet aperture. You recollect Herschel said that it was a good observing year, in which there were 100 hours fit for observing, and of the average of our hours I have not employed above 30. We have been for the last two years engaged in constructing a telescope of 6 feet aperture and 52 feet focus, and it would have been impossible to have bestowed the necessary attention upon it had we made a business of observing. That instrument is nearly finished, and I hope it will effect something for astronomy. The unequal refraction of the atmosphere will limit its powers, but how far remains to be ascertained.... Lady Rosse joins me in very kind remembrances and believe me to be,
Dear Mrs. Somerville,
Yours very truly and ever,
Rosse.