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The scheme of classification due to the Potsdam astro-physicist differed from Father Secchi's only in presenting his third and fourth types as subdivisions of the same order, and in inserting three subordinate categories; but their variety was "rationalised" by the addition of the seductive idea of progressive development.
The rays which were its messengers, admitted within the slit of Sir William Huggins's spectroscope, May 16, proved to be of a composition highly significant as to the nature of the catastrophe. The star which had already declined below the third magnitude showed what was described as a double spectrum. To the dusky flutings of Secchi's third type four brilliant rays were added.
From Father Secchi's and Professor Respighi's observations, 1869-71, were derived the first clear ideas on the subject, which have been supplemented and modified by the later researches of Professors Tacchini and Riccò at Rome and Palermo. The results are somewhat complicated, but may be stated broadly as follows.
"One observatory is so much like another that it does not seem worth while to describe Father Secchi's. Secchi had no staff, and no prescribed duties. The base of the observatory was the solid foundation of the old Roman building. The church was built in 1650, and the monastery in part at that time, certainly the dome of the room in which was the meridian instrument.
The registration of their spectra was sought to be made more distinctive than had previously been attempted, Secchi's first type being divided into four, his second into five subdivisions; but the differences regarded in them could be confidently established only for stars above the sixth magnitude.
Johnstone Stoney advocated, in 1867, the comparative youth of red stars; A. Ritter, of Aix-la-Chapelle, divided them, in 1883, into two squadrons, posted, the one on the ascending, the other on the descending branch of the temperature-curve, and corresponding, presumably, with Secchi's third and fourth orders of stars with banded spectra.
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