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 stars, so called, did not appear to be really fixed, but that they described small circles in the heavens each year. This fact greatly perplexed him, until at last he hit upon the true solution by taking into account the motion of the earth in its orbit, together with the fact that light had a finite velocity. This result showed that the light from the stars travelled with the same velocity as that which travelled from Jupiter's satellites. The Aberration of Light, as his discovery was termed, may be illustrated in the following way--Suppose that you are standing still, and that it is raining, the rain descending vertically on the umbrella that you hold up to cover you. As soon as you begin to walk, the rain-drops will apparently begin to slant, and if the walk is changed into a run, the greater apparently will be the slanting direction that the rain-drops take. In the same way, the rays of light from a star would fall vertically upon the earth if it were motionless, but as the