nish as the refractive
index increases. For water it is 481/2 deg., for flint glass 38 deg.41', and for
diamond 23 deg.42'. Thus all the light incident from two complete
quadrants, or 180 deg., in the case of diamond, is condensed into an
angular space of 47 deg.22' (twice 23 deg.42') by refraction. Coupled with its
great refraction, are the great dispersive and great reflective
powers of diamond; hence the extraordinary radiance of the gem, both
as regards white light and prismatic light.

Sec. 5. _Velocity of Light. Aberration. Principle of least Action._

In 1676 a great impulse was given to optics by astronomy. In that year
Olav Roemer, a learned Dane, was engaged at the Observatory of Paris
in observing the eclipses of Jupiter's moons. The planet, whose
distance from the sun is 475,693,000 miles, has four satellites. We
are now only concerned with the one nearest to the planet. Roemer
watched this moon, saw it move round the planet, plunge into Jupiter's
shadow, behaving like a lamp suddenly extinguished: then at the other
edge of the shadow he saw it reappear, like a lamp suddenly lighted.
The moon thus acted the part of a signal light to the astronomer, and
enabled him to tell exactly its time of revolution. The period between
two successive lightings up of the lunar lamp he found to be 42 hours,
28 minutes, and 35 seconds.

This measurement of time was so accurate, that having determined the
moment when the moon emerged from the shadow, the moment of its
hundredth appearance could also be determined. In fact, it would be
100 times 42 hours, 28 minutes, 35 seconds, after the first
observation.

Roemer's first observation was made when the earth was in the part of
its orbit nearest Jupiter. About six months afterwards, the earth
being then at the opposite side of its orbit, when the little moon
ought to have made its hundredth appearance, it was found unpunctual,
being fully 15 minutes behind its calculated time. Its appearance,
moreover, had been growing gradually later, as the earth retreated
towards the part of its orbit most distant from Jupiter. Roemer
reasoned thus: 'Had I been able to remain at the other side of the
earth's orbit, the moon might have appeared always at the proper
instant; an observer placed there would probably have seen the moon 15
minutes ago, the retardation in my case being due to the fact that the
light requires 15 minutes to travel from the place where my first
observation was