ESSAY ON THE VELOCITY OF ‘LIGHT. 149 
earth. ‘The sun’s parallax, calculated from the observations of the last transit 
of Venus over the disk of the sun in 1769, is fixed at 8.57 seconds; hence the 
distance of the sun from the earth is equal to 24,109 times the radius of the 
earth, or to 153,500,000 kilometres (or to 95,384,900 miles.) As this length 
is run over by the light in 8 minutes 18 seconds or in 498 seconds, we conclude 
that the velocity of light is 308,000 kilometres (or 191,391 miles) in one second. 
However, for some years several circumstances have conspireu to make us 
believe that the determination of 8.57 seconds given as the value of the sun’s 
parallax is too small, and that the parallax ought to be augmented by 2 quan- 
tity not less than the thirtieth of its value, which would elevate it to about 8.9 
seconds. From this increase in parallax:results a diminution in the earth’s dis- 
tance from the sun, and consequently in the distance gone over in 8 minutes 
18 seconds by the light; the velocity of light will therefore be reduced to a little 
less than 300,000 kilometres (or 186,420 miles) in a second. 'The next transit 
of Venus,* which will happen in 1874, cannot fail to set at rest all doubts which 
‘may yet remain on this point. 
THE EXPERIMENT PROJECTED BY ARAGO TO DETERMINE THE VELOCITY OF 
LIGHT. t 
The velocity of light, diseovered and determined by the labors of which we 
have just spoken, was entered as an established fact among the truths of sci- 
ence. Turther researches could at most only lead to a oveater precision in its 
determination, already approximated so closely by Bradley. The magnitude 
of that velocity, so great that in one second it runs over more than seven and a 
half times the circumference of the earth, would cause us to consider the 
dimensions of the earth as far too small to serve as a basis for an experimental 
determination of its value; the incomparably greater dimension of the earth’s 
annual orbit was not more than sufficient to reveal to Roémer, from his obser 
vations of the eclipses of Jupiter’s first satellite, that prodigious velocity, aie 
we would naturally think that the measure of the velocity of light, which. 
from its nature, really belongs to the province of experimental phy: sies, would 
never depart from the domain of pleted Nevertheless, as we will soon 
see, this recently did take place in the most brilliant manner 
The first step in this direction was the most difficult to waned: and it required 
all the daring of genius to attempt it. We find it in an experiment projected 
by Arago, and communicated to the Academy of Sciences of Paris during its 
mecting on the 3d of December, 1838. In the project it was not as yet pro- 
posed to measure the velocity of light, but simply to compare the velocities 
with which light moves in air, or in a liquid such as water, or bisulphide of 
carbon; it was proposed to find by experiment which of these two velocities 
was the greater, which would decide in an irrefutable manner st tae the two 
systems imagined by physicists to explain optical phenomena, viz: the system 
. 
* The observation ol the transit of the planet Venus across the disk of the sun is by far the 
most precise means that we can employ to determine the value of the parallax of the sun. 
t Before proceeding further in the perusal of the essay of M. Delaunay, it is necessary 
that all who have not given especial attention to the study of recent optical research, and 
who desire to appreciate the beauty and importance of the remainder of this essay, should 
understand wlty light should move faster in water than in air according to the emission 
theory, and slower in water than in air according to the undulatory theory. This is not 
explained by the author, and without this knowl ledge it is impossible to appreciate the 
excellence of these classical experiments of Arago, of b'izeau, and of Foucault. 
We would advise the above class of readers to study the points here spoken of in the 
‘*Lectures on the Undulating Theory of Light,” by Professor Barnard, Smithsonian R eport 
for 1862. in the admirable *Traité de Paysique, "by Dacuin, Paris, 1862, and in Pouillet’s 
‘*Traite de Physique,” will be found detailed accounts of the apparatus mentioned in this 
essay, illustrated with engravings. The original memoirs in the transactions of the Academy 
of Sciences of Paris shou!d also be consulied.— Tr. 
