146 REPORT—1868. 
observed to be continually changing, at one time increasing, at another time 
diminishing. The careful study of these variable stars by numerous ob- 
servers has shown that their continual changes do not take place in an 
uncertain or irregular manner. The greater part of these remarkable objects 
wax and wane in accordance with a fixed law of periodic variation which is 
peculiar to each. 
The speaker stated that he had been seeking for some time to throw light 
upon this strange phenomenon by means of observation of their spectra. If 
in any case the periodic variation of brightness is associated with physical 
changes occurring in the star, we might obtain some information by means of 
the prism. Again, if the diminution in brightness of a star should be caused 
by the interposition of a dark body, then in that case, if the dark body be 
surrounded with an atmosphere, its presence might possibly be revealed to us 
by the appearance of additional lines of absorption in the spectrum of the 
star when at its minimum. Some small changes haye been suspected, but 
further observations are required before any conclusion can be with certainty 
deduced from them. 
TEMPORARY STARS. 
With the variable stars modern opinion would associate the remarkable 
phenomena of the so-called new stars which occasionally, but at long 
intervals, have suddenly appeared in the sky. But in no case has a per- 
manently bright star been added to the heavens. The splendour of all these 
objects was temporary only, though whether they died out or still exist as 
extremely faint stars is uncertain. In the case of the two modern temporary 
stars, the one seen by Mr. Hind in 1845, and the bright star recently observed 
in Corona, though they have lost their ephemeral glory, they still continue 
as stars of the 10th and 11th magnitude. 
The old theories respecting these strange objects must be rejected. We 
cannot believe, with Tycho Brahe, that objects so ephemeral are new creations, 
nor with Riccioli, that they are stars brilliant on one side only, which haye 
been suddenly turned round by the Deity. The theory that they have sud- 
denly darted, towards us with a velocity greater than that of light, from a 
region of remote invisibility, will not now find supporters. 
On the 12th of May last a star of the 2nd magnitude suddenly burst forth 
in the constellation of the Northern Crown. Thanks to the kindness of the 
first discoverer of this phenomenon, Mr. Birmingham, of Tuam, the speaker 
was enabled, conjointly with Dr. Miller, to examine the spectrum of this star 
on the 16th of May, when it had not fallen much below the 3rd magnitude. 
The spectrum of this star consists of two distinct spectra. One of these is 
formed of four bright lines. The other spectrum is analogous to the spectra 
of the sun and stars. é 
These two spectra represent two distinct sources of light. Each spectrum 
is formed by the decomposition of light, which is independent of the light 
which gives birth to the other spectrum. 
The continuous spectrum, crowded with groups of dark lines, shows that 
there exists a photosphere of incandescent solid or liquid matter. Further, 
that there is an atmosphere of cooler vapours, which give rise, by absorption, 
to the group of dark lines. 
So far the constitution of this object is analogous to that of the sun and 
stars; but, in addition, there is the second spectrum, which consists of bright 
lines. There is therefore a second and distinct source of light, and this must 
be, as the character of the spectrum shows, luminous gas. Now the two 
