.[1497]
Of particular interest is the emergence in the star-spectrum
photographed by Von Gothard of an ultra-violet line originally
discovered at Tulse Hill in the Orion nebula, which is also very strong
in the Lyra annular nebula.

Obviously, then, the physical constitution of Nova Aurigae became
profoundly modified during the four months of its invisibility. The
spectrum of February was or appeared compound; that of August was
simple; it could be reasonably associated only with a single
light-source. Many of the former brilliant lines, too, had vanished, and
been replaced by others, at first inconspicuous or absent. As a result,
the solar-prominence type, to which the earlier spectrum had seemed to
conform, was completely effaced in the later. The cause of these
alterations remains mysterious, yet its effects continue. The chromatic
behaviour of the semi-extinct Nova, when scrutinised with great
refractors, shows its waning light to be distinctly nebular.[1498] Like
nearly all its congeners, the star is situated in the full stream of the
Milky Way, and we learn without surprise that micrometrical measures by
Burnham and Barnard[1499] failed to elicit from it any sign of
parallactic shifting. It is hence certain that the development of light,
of which the news reached the earth in December, 1891, must have been on
a vast scale, and of ancient date. Nova Aurigae at its maximum assuredly
exceeded the sun many times in brightness; and its conflagration can
scarcely have occurred less, and may have occurred much more, than a
hundred years ago.

By means of the photographic surveys of the skies, carried on in both
hemispheres under Professor Pickering's superintendence, such amazing
events have been proved to be of not infrequent occurrence. Within six
years five new stars were detected from Draper Memorial, or chart-plates
by Mrs Fleming, besides the retrospective discovery of a sixth which had
rapidly burnt itself out, eight years previously, in Perseus.[1500] Nova
Normae was the immediate successor of Nova Aurigae; Nova Carinae and Nova
Centauri lit up in 1895, the latter in a pre-existent nebula; Nova
Sagittarii and Nova Aquilae attained brief maxima in 1898 and 1899
respectively. Now, three out of these five stars reproduced with
singular fidelity the spectrum of Nova Aurigae; they displayed the same
brilliant rays shadowed, invariably on their blue sides, by dark ones.
Palpably, then, the arrangement was systematic