ESSAY ON THE VELOCITY OF LIGHT. 147 
‘cause, irrespective of each individual star, is explained most satisfactorily by 
the hypothesis of the progressive transmission of light, and the result is in per- 
feet accord with the deductions of Roémer, from the observations of the eclipses 
of Jupiter’s satellites. Such an agreement of circumstances does not allow of 
the least doubt as to the truth of Roémer’s conception. ; 
We will remark, in passing, that aberration, which is a result due to the com- 
bination of the velocity of light with the velocity of the earth, in its annual 
movement around the sun, furnished at the same time a proof in favor of the 
system of Copernicus—a proof which had, up to that period, been vainly sought 
in the annual parallax of the stars. 
‘This phenomenon, discovered, studied, and so well explained by Bradley, 
had not remained entirely unknown before his time. In fact, in the work of 
Picard, entitled Voyage d’ Uranibourg, speaking cf the different values which 
Tycho had found, for the height of the pole at his observatory of Uranibourg, 
we read the following passage: 
‘* Besides that, there is an obstacle in the way of the use of the polar star, (for determining 
the height of the pole,) which, from one season to another, experiences certain variations 
which l'yeho had not remarked, and which I observed nearly ten years ago, which consists 
in this—that, although the polar star approaches continually the pole* by about twenty 
seconds, it nevertheless happens that towards the month of April the inferior meridian height 
of that star becomes less, by several seconds, than it appeared at the preceding winter sol- 
stice, instead of which it should have been greater by five seconds, and, consequently, in the 
months of August and September its superior meridian height is found to be nearly that which 
it was observed to have in winter, and sometimes even greater, although it ought to have 
diminished from ten to fifteen seconds; but, finally, towards the end of the year all is com- 
aig td so that the polar star appears nearer to the pole by about twenty seconds than it 
was before. 
There is no denying that we have here the annual change of position due to 
aberration. Picard adds, a little further on: 
‘“To tell the truth, I have not as yet been able to imagine any explanation that satisties 
me.” 
Delambre, who cites these passages in his Histoire de l’ Astronomie Moderne, 
t. ii, page 616, accompanies them with the following reflection : 
“When Roémer, brought into 'rance by Picard, measured the velocity of light, he little 
imagined, notwithstanding the identity of the annual period, that his discovery had anything 
in common with those variations which had for so long troubled Picard, who, no doubt, had 
often spoken of them.” 
In the two phenemena, of which we have successively spoken, the progres- 
sive transmission of light does not play the same part. If the observations of 
the eclipses of Jupiter’s satellites unveiled to Roémer a periodic perturbation, 
agreeing with the successive augmentations and diminutions of the distance of 
Jupiter from the earth, that is owing to the time the light employs to traverse 
the earth’s orbit; the velocity of the earth has nothing to do with the ampli- 
tude of the phenomenon, which depends entirely on the dimensions of its orbit. 
_In the phenomenon of aberration, on the contrary, it is the velocity of the earth 
which is principally concerned ; and it is of little consequence whether its orbit 
be small or large. If we transport ourselves in thought to the planet Mars, 
which describes an orbit of larger dimensions than the earth, but which at the 
same time moves with a less velocity, we will sec that the two phenomena of 
which we speak will vary as to their magnitude in an inverse manner: the 
eclipses of the first satcllite of Jupiter will appear to experience a greater per- 
turbation, and the aberration of the stars will be less. The contrary will happen 
if we place ourselves on the planet Venus, which has a greater velocity of 
translation than the earth, and runs over a smaller orbit: the aberration of the 
* On account of the precession of the equinoxes. 
