iking of the match, the travelling of the light to the walls, and its
return to our eyes. The scale of our terrestrial phenomenon is far too
small to render this interval perceptible. But those who accept the theory
above mentioned regarding the appearances round Nova Persei (although
there are some who discredit it) believe that we have in this case an
illustration of just this phenomenon of light echoes, on a scale large
enough to be easily visible. They think that, surrounding the central star
which blazed up so brightly in February 1901, there was a vast dark
nebula, of which we had no previous knowledge, because it was not shining
with any light of its own. When the star blazed up, the illumination
travelled from point to point of this dark nebula and lighted it up; but
the size of the nebula was so vast that, although the light was travelling
with the enormous velocity of 200,000 miles per second, it was not until
months afterwards that it reached different portions of this nebula; and
we accordingly got news of the existence of this nebula some months after
the news reached us of the central conflagration, whatever it was. Remark
that all we can say is that the news of the nebula reached us _some months
later_ than that of the outburst. The actual date when either of the
actual things happened, we have as yet no means of knowing; it may have
been hundreds or even thousands of years ago that the conflagration
actually occurred of which we got news in February 1901, the light having
taken all that time to reach us from that distant part of space; and the
light reflected from the nebula was following it on its way to us all
these years at that same interval of a few months.

[Sidenote: An objection.]

Now, let me refer before leaving this point to the chief objection which
has been urged against this theory. It has been maintained that the
illumination would necessarily appear to travel outwards from the centre
with an approach to uniformity, whereas the observed rate of travel is not
uniform, and has been even towards the centre instead of away from it;
which would seem as though portions of the nebula more distant from the
centre were lighted up sooner than those closer to it. By a simple
illustration from our solar system, we shall see that these curious
anomalies may easily be explained. Let us consider for simplicity two
planets only, say the Earth and Saturn. We know that Saturn travels round
the sun in an orb