138 ESSAY ON THE VELOCITY OF LIGH?. 
DISCOVERY OF THE VELOCITY OF LIGHT, BY ROBMER. 
During a long period we had no knowledge of the velocity of light. On ac- 
count of the extreme greatness of that velocity, the phenomena of light pro- 
duced on the earth being perceived immediately at considerable distances on 
the surface of the globe, it was usual to consider the passage of light through 
space as absolutely instantaneous.* The honor of the discovery of the pro-_ 
gressive transmission of light and the determination of the value of its velocity 
is due to Roémer, a Danish astronomer who was induced by Picard to settle in 
France in 1672. He made the important discovery by examining and discuss- 
ing with care the observations on the eclipse of the first satellite of Jupiter. 
It is well known that Jupiter, the greatest of the planets which, like the earth, 
circulates around the sun, is accompanied by four moons or satellites. These 
satellites move around the planet in the same manner as the planets move 
around the sun, conformably to the laws of Kepler; their elliptical orbits are 
in planes slightly inclined to the plane of the orbit of Jupiter. The planets 
and their satellites have not a light of their own, and we can only see them 
when they are illuminated by the sun. If any obstacle prevents the light from 
falling on one of these bodies, it becomes invisible, or, in other words, it is 
eclipsed. This is what happens frequently to each of the satellites of Jupiter. 
The satellite in circulating around its planet comes to a portion of its orbit which, 
relatively to the sun, is behind Jupiter; the planet intercepting the rays of the 
sun, the satellite is eclipsed for a certain time: this is a phenomenon entirely 
simidar to the eclipses of the moon which we from time to time have the oppor- 
tunity of observing. If we imagine a cone enveloping at the same time the 
sun and Jupiter, so that the surface of the cone is tangential to the two spheres, 
the light of the sun cannot reach any point of the space situate within the cone 
and beyond Jupiter; this space is what we call the wmbra or umbral cone of 
the planet. A satellite is eclipsed every time that in circulating around the 
planet it penetrates this umbral cone; and the eclipse ceases the instant it has 
traversed it. On account of the large transverse dimensions of the umbral 
cone of Jupiter due to the size of the planet, and also on account of the slight 
inclination of the orbits of its satellites to the orbit the planet describes around 
the sun, these satellites penctrate at each of their revolutions the umbral cone; 
there is an exception alone for the fourth satellite, (that which is the furthest 
from the planet,) which passes sometimes above or below the umbral cone with- 
out penetrating it, when it passes in the parts of its orbit the most distant from 
the plane of the orbit of Jupiter. 
The eclipses of the first satellite of Jupiter (that which is nearest to the 
planet) are much more frequent than those of the other satellites, on account 
of the rapidity with which it describes its orbit. These eclipses are repeated 
at intervals of about forty-two hours and a half. Also the eccentricity of the 
orbit of this satellite being insensible, its movement around the planet is circular 
and uniform: there necessarily results a great regularity in the occurrence of 
the successive eclipses. ‘Ihese phenomena are easily observed from the earth, 
* The experiment of the lights explained above was made by Galileo, and did not give any 
result. Subsequently, Descartes thought of a celestial phenomenon which he imagined would 
be affected by the progressive transmission of light. He saw that if light occupies a certain 
time to traverse the distance which separates us from the moon, then that satellite, at the 
moment it is eclipsed by the interposition of the earth between the sun and the moon, should 
not appear to occupy in the heavens a position diametrically opposite to the sun; it should, 
on the contrary, appear notably distant from that position. Observation did not show any- 
thing similar, and he concluded that light was transmitted instantaneously through space. 
These negative results only prove that the velocity of light is too great to produce, in these 
circumstances, effects appreciable by the means of observation employed. They can, at the 
most, as Huyghens remarks, furnish an inferior limit to the velocity of light, and do not at 
all prove that this velocity is infinite. 
