146 ESSAY ON THE VELOCITY OF LIGHT. 
his being 241,) judging, from the experience which I already had, that this radius would be 
long enough to adjust the instrument to a sufficient degree of exactness, and I have had no 
reason since to change my opinion, for, from all the trials I have yet made, I am very well 
satisfied that, when it is carefully rectified, its situation may be securely depended upon to 
half a second. As the place where my instrument was to be hung in some measure deter- 
mined its radius, so did it also the length of the arch, or limb, on which the divisions were 
made to adjust it, for the arch could not conveniently be extended further than to reach to 
about six and one-fourth degrees on each side my zenith. This, indeed, was sufficient, since 
it gave me an opportunity of making choice of several stars, very different both in magnitude 
and situation, there being more than two huadred inserted in the British catalogue that may 
be observed with it. I needed not to have extended the limb so far, but that I was willing 
to take in Capella, the only star of the first magnitude that comes so near my zenith. 
‘‘ My instrument being fixed, I immediately began to observe such stars as I judged most 
proper to give me light into the cause of the motion already mentioned. There was variety 
enough of small ones, and not less than twelve that I could observe through all seasons of 
the year, they being bright enough to be seen in the day-time, when nearest the sun.” 
Bradley here enters into some details on the results which he obtained in the 
observations on different stars. He then adds: 
‘‘When the year was completed I began to examine and compare my observations, and 
having pretty well satisfied myself as to the general laws of the phenomena, I then endea- 
vored to find out the cause of them. I was already convinced that the apparent motion of 
the stars was not owing to a mutation of the earth’s axis. The next thing that offered itself 
was an alteration in the direction of the plumb-line with which the instrument was constantly 
rectified; but this, upon trial, proved insufficient. Then I considered what refraction might 
do; but here, also, nothing satisfactory occurred. At last I conjectured that all the pheno- 
mena hitherto mentioned proceeded from the progressive motion of light and the earth’s annual 
motion in its orbit, for I perceived that, if light was propagated in time, the apparent place 
of a fixed object would not be the same when the eye is at rest as when it is moving in any 
other direction than that of the line passing through the eye and object, and that when the 
eye is moving in different directions the apparent place of the object would be different.”’ 
Bradley examines then, in detail, the apparent positions which the stars 
should have from the combination of the velocity of the earth with the velocity 
of the propagation of light through space, and he finds a perfect agreement 
between the results thus arrived at and those furnished by observation. ‘This 
change in position of the stars which he had studied with such care was there- 
fore explained, and he was enabled also, from the magnitude of these changes 
of position, to determine the ratio of the velocity of light to the velocity of 
translation of the earth. In making this determination from the data given by 
his observations on the various stars he had studied, he arrives at very con- 
cordant results, whence he concludes that the greater axis of the ellipse of aber- 
ration, which is the same for all the stars, has an amplitude of forty and a half 
seconds. It follows that the velocity of light is 10,188 times greater than the 
velocity of the earth, (the mean velocity being understood,) so that the light 
ought to take eight minutes and thirteen seconds to come from the sun to us. 
Bradley, referring here to the discovery of Roémer, says: 
“Tt is well known that Mr. Roémer, who first attempted to account for an apparent 
inequality in the times of the eclipses of Jupiter’s satellites by the hypothesis of the progres- 
sive motion of light, supposed that it spent about eleven minutes of time in its passage from 
the sun to us, but it hath since been concluded by others, from the like eclipses, that it is 
propagated as far in about seven minutes. The velocity of light, therefore, deduced from 
the foregoing hypothesis, is, as it were, a mean betwixt what had at different times been 
determined from the eclipses of Jupiter’s satellites.” 
After the discovery .of Bradley, the progressive transmission of light became 
an incontestable fact. Here we do not consider merely the perturbation affeet- 
ing the position of a single star as in the phenomenon discovered by Roémer, 
a perturbation that we might, in truth, regard as really existing in the motion 
of the satellite, although we might not be able to give the ultimate cause. But 
in the new phenomenon discovered by Bradley we observe all the stars sub- 
jected to a perturbation of the same nature, all following the same laws, and 
not varying from each other, but by reason of their different positions relatively 
to the ecliptic. This general perturbation, evidently owing to a single general 
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