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On the Tails of Comets. 37 
Arago, in speaking of the then anticipated return of Halley’s 
comet in 1835, makes the following remark: “It appears proba- 
ble that in describing their immense orbits, comets at each revo- 
lution, dissipate in space all the matter which when they are near 
the perihelion, is detached from the envelop forming the tail ; it 
is therefore very possible that in time some of them may be en- 
tirely dissipated.” But these views were not confirmed by the 
appearance of Halley’s comet in 1835, and Arago has with a 
very becoming candor promptly acknowledged this fact. “If 
_ the reader,” says he, “will take the trouble to compare what I 
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record of the comet of 1835 with the circumstances of its former 
apparition, he certainly will not find in this collection of phenom- 
ena the proof that Halley’s comet is gradually diminishing. I 
will even say that if, in a matter so delicate, observations made 
at very different periods of the year, will authorize any positive 
deduction, that which would most distinctly result from the two 
passages of 1759 and 1835, would be that the comet had increas- 
ed in size during that interval. I ought to seize with more ea- 
gerness, this occasion to combat an error extensively accredited, 
(a belief in the constant wasting away of comets, ) because I be- 
lieve Ihave somewhat senteiented to its dissemination.” 
The truth is, as I apprehend, that the data on which this 
conjecture was based, are r otobeuty false,and the tails of comets, 
_if the subject is properly investigated, will not be found to con- 
sist of matter at all that has the least connection with the comet, 
_ but formed by the sun’s rays slightly refracted by the nucleus in 
: traversing the envelop of the comet, and uniting in an infinite — 
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number of points beyond it, throwing a stronger than ordinary 
light on the ethereal medium, near to or more remote from the 
comet, as the ray fromits relative position and direction is more or 
less refracted. 
It is not important to the truth of this hypothesis whether the 
nucleus be a solid mass or not, so that it be more dense than the 
surrounding nebulosity, nor yet that the tail be projected in an ex- 
act line with the radius vector of the sun and comet, so that it be 
nearly so. It is however important to its truth that an ethereal 
medium should exist, otherwise the reflection of these points of 
light would be impossible ; also that the comet should assume the 
tail as it approaches the sun, and that it should progressively in- 
crease in length and brilliancy, the light of the sun increasing in 
