"The destined course of whirling worlds to trace, To plot the highways of the universe, And hear the morning stars their song rehearse, And find the wandering comet in his place; This is the triumph written in his face, And in the gleaming eye that read the sun Like open book, and from the spectrum won The secrets of immeasurable space."[13] CHAPTER II Mercury As the elongation of Mercury from the sun seldom exceeds 18°, it is a difficult object, at least in this country, to see without a telescope. As the poet says, the planet-- "Can scarce be caught by philosophic eye Lost in the near effulgence of its blaze." Tycho Brahé, however, records several observations of Mercury with the unaided vision in Denmark. It can be occasionally caught with the naked eye in this country after sunset, when it is favourably placed for observation, and I have so seen it several times in Ireland. On February 19, 1888, I found it very visible in strong twilight near the western horizon, and apparently brighter than an average star of the first magnitude would be in the same position. In the clear air of the Punjab sky I observed Mercury on November 24-29, 1872, near the western horizon after sunset. Its appearance was that of a reddish star of the first magnitude. On November 29 I compared its brilliancy with that of Saturn, which was some distance above it, and making allowance for the glare near the horizon in which Mercury was immersed, its brightness appeared to me to be quite equal to that of Saturn. In June, 1874, I found it equal to Aldebaran, and of very much the same colour. Mr. W. F. Denning, the famous observer of meteors, states that he observed Mercury with the naked eye about 150 times during the years 1868 to 1905.[14] He found that the duration of visibility after sunset is about 1{h} 40{m} when seen in March, 1{h} 30{m} in April, and 1{h} 20{m} in May. He thinks that the planet is, at its brightest, "certainly much brighter than a first magnitude star."[15] In February, 1868, he found that its brightness rivalled that of Jupiter, then only 2° or 3° distant. In November, 1882, it seemed brighter than Sirius. In 1876 it was more striking than Mars, but the latter was then "faint and at a considerable distance from the earth." In 1878, when Mercury and Venus were in the same field of view of a telescope, Nasmyth found that the surface brightness (or "intrinsic brightness," as it is called) of Venus was at least twice as great as that of Mercury; and Zöllner found that from a photometric point of view the surface of Mercury is comparable with that of the moon.