housand times as long
it would hardly enable manual dexterity to close the aperture. Yet a
shutter can be constructed which shall be sufficiently delicate for the
purpose.

[Illustration: Fig. 63.--Mode of Measuring the Velocity of Light.]

The principle of this beautiful method will be sufficiently obvious from
the diagram on this page (Fig. 63), which has been taken from Newcomb's
"Popular Astronomy." The figure exhibits the lantern and the observer,
and a large wheel with projecting teeth. Each tooth as it passes round
eclipses the beam of light emerging from the lantern, and also the eye,
which is of course directed to the mirror at the distant station. In the
position of the wheel here shown the ray from the lantern will pass to
the mirror and back so as to be visible to the eye; but if the wheel be
rotating, it may so happen that the beam after leaving the lantern will
not have time to return before the next tooth of the wheel comes in
front of the eye and screens it. If the wheel be urged still faster, the
next tooth may have passed the eye, so that the ray again becomes
visible. The speed at which the wheel is rotating can be measured. We
can thus determine the time taken by one of the teeth to pass in front
of the eye; we have accordingly a measure of the time occupied by the
ray of light in the double journey, and hence we have a measurement of
the velocity of light.

It thus appears that we can tell the velocity of light either by the
observations of Jupiter's satellites or by experimental enquiry. If we
take the latter method, then we are entitled to deduce remarkable
astronomical consequences. We can, in fact, employ this method for
solving that great problem so often referred to--the distance from the
earth to the sun--though it cannot compete in accuracy with some of the
other methods.

The dimensions of the solar system are so considerable that a sunbeam
requires an appreciable interval of time to span the abyss which
separates the earth from the sun. Eight minutes is approximately the
duration of the journey, so that at any moment we see the sun as it
appeared eight minutes earlier to an observer in its immediate
neighbourhood. In fact, if the sun were to be suddenly blotted out it
would still be seen shining brilliantly for eight minutes after it had
really disappeared. We can determine this period from the eclipses of
Jupiter's satellites.

So long as the satellite is shining it radiates a strea