r
example, until it reaches the body which it lights up. The velocity of
the light waves, however, is so great, that it is almost impossible to
give any comparative idea of their rate of transmission. The velocity of
the light waves was first established by Roemer, a Danish astronomer, in
1675. He ascertained the velocity of light by observations made on the
satellites of Jupiter. His methods of reasoning can easily be understood
by reference to the following diagram.

[Illustration: Fig: 8.]

Let _S_ represent the sun, and _A_ and _B_ the orbit of the earth round
the sun; _C_ _E_ _D_ part of Jupiter's orbit round the sun; while _D_
_E_ _F_ represents the orbit of Jupiter's satellite. When the earth,
Jupiter, and the satellite are in a straight line with each other, the
satellite suffers an eclipse through passing into the shadow thrown by
Jupiter. Now Roemer found that there was a difference in the time of the
eclipse when the earth was at _B_, that is, when it was nearest to
Jupiter, and when it was at _A_, which is that part of the earth's orbit
furthest away from Jupiter. That difference was accounted for by the
fact, that when the earth was at _A_ the light had to travel further
from Jupiter than when the earth was nearest to Jupiter, that is at
point _B_. Thus, when the earth was nearest to Jupiter, the light had a
shorter journey to travel than when it was furthest from Jupiter. The
difference, he found, was about 16 minutes, and he reasoned that this
difference was caused by the light having to cross the earth's orbit
from _B_ to _A_, in its longer journey, than when it only had to reach
the earth at _B_. The mean distance of the earth from the sun, that is,
the radius of the earth's orbit, is about 92-1/2 million miles, so that
the diameter of the earth's orbit is about 185,000,000 miles, and if it
takes about 16 minutes for light to traverse this distance, we find that
light has a velocity, according to Roemer, of 192,500 miles per second.
The result, however, arrived at by Roemer was not generally accepted at
that time, and it was not till 1728 that Bradley discovered what is
known as the Aberration of Light, and from that discovery proved that
light was not transmitted instantaneously through space, but that it was
transmitted with finite velocity; and that that velocity corresponded
fairly well with the velocity given by Roemer. Bradley, in his
astronomical observations, noticed that some of the fixed star