The eclipse of August 18, 1868, the total phase of which lasted nearly six minutes, was visible in India, and drew thither a large concourse of astronomers. In this eclipse the spectroscope came to the front, and showed that both the prominences, and the chromospheric layer from which they rise, are composed of glowing vapours—chief among which is the vapour of hydrogen. The direct result of the observations made on this occasion was the spectroscopic method of examining prominences at any time in full daylight, and without a total eclipse. This method, which has given such an immense impetus to the study of the sun, was the outcome of independent and simultaneous investigation on the part of the French astronomer, the late M. Janssen, and the English astronomer, Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscent of the discovery of Neptune. The principles on which the method was founded seem, however, to have occurred to Dr. (now Sir William) Huggins some time previously.
The eclipse of December 22, 1870, was total for a little more than two minutes, and its track passed across the Mediterranean. M. Janssen, of whom mention has just been made, escaped in a balloon from then besieged Paris, taking his instruments with him, and made his way to Oran, in Algeria, in order to observe it; but his expectations were disappointed by cloudy weather. The expedition sent out from England had the misfortune to be shipwrecked off the coast of Sicily. But the occasion was redeemed by a memorable observation made by the American astronomer, the late Professor Young, which revealed the existence of what is now known as the "Reversing Layer." This is a shallow layer of gases which lies immediately beneath the chromosphere. An illustration of the corona, as it was seen during the above eclipse, will be found on [Plate VII.] (A), p. 142.
In the eclipse of December 12, 1871, total across Southern India, the photographs of the corona obtained by Mr. Davis, assistant to Lord Lindsay (now the Earl of Crawford), displayed a wealth of detail hitherto unapproached.
The eclipse of July 29, 1878, total across the western states of North America, was a remarkable success, and a magnificent view of the corona was obtained by the well-known American astronomer and physicist, the late Professor Langley, from the summit of Pike's Peak, Colorado, over 14,000 feet above the level of the sea. The coronal streamers were seen to extend to a much greater distance at this altitude than at points less elevated, and the corona itself remained visible during more than four minutes after the end of totality. It was, however, not entirely a question of altitude; the coronal streamers were actually very much longer on this occasion than in most of the eclipses which had previously been observed.
The eclipse of May 17, 1882, observed in Upper Egypt, is notable from the fact that, in one of the photographs taken by Dr. Schuster at Sohag, a bright comet appeared near the outer limit of the corona ([see Plate I.], p. 96). The comet in question had not been seen before the eclipse, and was never seen afterwards. This is the third occasion on which attention has been drawn to a comet merely by a total eclipse. The first is mentioned by Seneca; and the second by Philostorgius, in an account of an eclipse observed at Constantinople in A.D. 418. A fourth case of the kind occurred in 1893, when faint evidences of one of these filmy objects were found on photographs of the corona taken by the American astronomer, Professor Schaeberle, during the total eclipse of April 16 of that year.
The eclipse of May 6, 1883, had a totality of over five minutes, but the central track unfortunately passed across the Pacific Ocean, and the sole point of land available for observing it from was one of the Marquesas Group, Caroline Island, a coral atoll seven and a half miles long by one and a half broad. Nevertheless astronomers did not hesitate to take up their posts upon that little spot, and were rewarded with good weather.
The next eclipse of importance was that of April 16, 1893. It stretched from Chili across South America and the Atlantic Ocean to the West Coast of Africa, and, as the weather was fine, many good results were obtained. Photographs were taken at both ends of the track, and these showed that the appearance of the corona remained unchanged during the interval of time occupied by the passage of the shadow across the earth. It was on the occasion of this eclipse that Professor Schaeberle found upon his photographs those traces of the presence of a comet, to which allusion has already been made.
Extensive preparations were made to observe the eclipse of August 9, 1896. Totality lasted from two to three minutes, and the track stretched from Norway to Japan. Bad weather disappointed the observers, with the exception of those taken to Nova Zembla by Sir George Baden Powell in his yacht Otaria.
The eclipse of January 22, 1898, across India viâ Bombay and Benares, was favoured with good weather, and is notable for a photograph obtained by Mrs. E.W. Maunder, which showed a ray of the corona extending to a most unusual distance.