.]

[Sidenote: Due to travelling illumination.]

The second point to which I will draw your attention is from the history
of the other Nova just mentioned--Dr. Anderson's New Star of 1901. In this
instance it is not the history previous to discovery, but what followed
many months after discovery, that was of engrossing interest; and again
Yerkes Observatory, with its magnificent equipment, played an important
part in the drama. When, with its giant reflecting telescope, photographs
were taken of the region of Nova Persei after it had become comparatively
faint, it was found that there was an extraordinarily faint nebulosity
surrounding the star. Repeating the photographs at intervals, it was
found that this nebulosity was rapidly changing in shape. "Rapidly" is, of
course, a relative term, and a casual inspection of two of the photographs
might not convey any impression of rapidity; it is only when we come to
consider the enormous distance at which the movements, or apparent
movements, of the nebulae must be taking place that it becomes clear how
rapid the changes must be. It was not possible to determine this distance
with any exactness, but limits to it could be set, and it seemed probable
that the velocity of the movement was comparable with that of light. The
conclusion suggested itself that the velocity might actually be identical
with that of light, in which case what we saw was not the movement of
actual matter, but merely that of illumination, travelling from point to
point of matter already existing.

[Illustration:

SEPT. 20, 1901 NOV. 13, 1901
IX--NEBULOSITY ROUND NOVA PERSEI
(_From photographs taken at the Yerkes Observatory by G. W. Ritchey._)]

[Sidenote: When did it all happen?]

An analogy from the familiar case of sound may make clearer what is meant.
If a loud noise is made in a large hall, we hear echoes from the walls.
The sound travels with a velocity of about 1100 feet per second, reaches
the walls, is reflected back from them, and returns to us with the same
velocity. From the interval occupied in going and returning we could
calculate the distance of the walls. The velocity of light is so enormous
compared with that of sound that we are usually quite unable to observe
any similar phenomenon in the case of light. If we strike a match in the
largest hall, all parts of it are illuminated so immediately that we
cannot possibly realise that there was really an interval between the
str