ESSAY ON THE VELOCITY OF LIGHT. 141 
consequently 11 minutes to run over the distance which separates us from that 
body. He published his discovery in a memoir presented to the Academy of 
Sciences of Paris in 1675, to which he had been elected a member a short time 
after his arrival in France. The following is the account given in the Histoire 
de l’ Académie, t. i, p. 213, année 1676 : 
“Tt was only from this vast mass of observations that we commenced to see a truth of 
physics hitherto unknown to all philosophers, and so ignored that the contrary was almost a 
fixed principle. 
‘‘4 great number of very exact calculations having been made of the revolutions of the 
first satellite of Jupiter, and consequently of all its eclipses caused by the umbra-of Jupiter, 
he found that at certain times, it came out of the shadow a few minutes too late, and at other 
times sooner than it should have done, and he could not account for that variation by known 
principles. In comparing these times with each other, M. Roémer saw that the satellite 
came too late from the shadow always when the earth, in its annual movement, was going 
away from Jupiter, and too soon when it was approaching the planet. From these facts M. 
Roémer began to form the ingenious conjecture that light requires an appreciabie time to 
traverse space. That granted, if the satellite appeared to come out of the shadow too late 
when we were furthest off from it, it did not follow that it really did emerge too late, but its 
light took longer to come to us, for, so to speak, we had run away from it. On the contrary, 
when we went to meet it, the time of the satellite in the shadow should appear shorter. 
“*To test the truth of this idea, he calculated what differences in the emersions of the satel- 
lite corresponded to different distances of the earth, and he found that the light was retarded 
eleven minutes for a length equal to the distance of the earth from thesun. From that datum 
he announced to the Academy, in the beginning of September, that if his supposition was 
correct, an emersion of the fwst satellite, which would take place on the 16th of November 
following, would happen ten minutes later than it should according to the ordinary cal- 
culation. 
‘*The event agreed with the prediction of M. Roémer. Notwithstanding this success, as 
the idea was very new, it was not at first generally received. The savans were cautious not 
to be led astray by the charms of novelty. The satellite has not for the centre of its motion 
the centre of Jupiter. Moreover, its revolutions are more rapid when it is nearer the sun, 
and this should produce irregularities in its motion. But these irregularities do not exactly 
follow those just mentioned. They even imagined another astronomical hypothesis, which 
would fulfil all the conditions, but it was too unlike anything hitherto known of the heavens. 
It might satisfy the calculation, but it had not that probability which could satisfy the minds 
*“We must, therefore, admit the retardation of light as probable, according to physics, 
even though not proved by astronomy.”’ 
The admirable discovery of Roémer was made known in an article pub- 
lished in the Journal des Savants of Mouday, December 7, 1676, under the 
title of Demonstration touchant le mouvement de la lumiere trouvé, par M. 
Roémer, del’ Academie Royale des Sciences. ‘This article, accompanied by a 
figure, gives a clear and sufficiently detailed explanation of the influence of 
the progressive transmission of light on the observations of the eclipses of 
Jupiter’s satellites. 
CONFIRMATION OF THE IDEAS OF ROEMER BY THE DISCOVERY OF THE ABER- 
RATION OF LIGHT, BY BRADLEY. 
The ideas of Roémer on the progressive transmission of light through space, 
and the explanation that he had thence deduced of the alternate delays and 
accelerations in the immersions and emersions of the first satellite of Jupiter, 
although generally admitted by savans, were not, however, accepted without 
dispute. ‘The point in consideration was the periodic perturbation in the move- 
ment of a single celestial body—the satellit.e -According to the observations 
this body was sometimes in advance of and sometimes behind the position it 
ought to occupy according to the preconceived ideas of the nature of its 
motion; instead of attributing these delays and accelerations to the change of 
position of the observer and to the progressive transmission of light, could it 
not be regarded as indicating a real perturbation in the movement of the satel- 
lite, due to a cause not yet discovered? The ideas of Roémer required, there- 
fore, a confirmation which would entirely relieve them of doubt. This con- 
firmation was given in a remarkable manner some time after. It was fur- 
