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. ae Remarks on the Tails of Comets. 
steady light, ‘or remain at once luminous and stationary, in any 
form in the pure air.”* aa 
So much for the theory of Hamilton, and the reasoning of our - 
author, whose conjectures must be established before they can be 
entitled to the character of-objections. There are moreover some 
difficulties which have presented themselves to my own mind, 
and may also have been noticed by others. Thus, one would 
suppose that the bending of the tail towards the region which 
the comet is leaving, might be more than adequate to answer the 
end of aberration; but in estimating this, the position of the tail 
relative to the observer, as well as its length, must be taken into 
the account. When the tail lies oblique to the line of vision, the 
extremity may be many millions of miles more remote than the 
nucleus, and some minutes of time may elapse after the arrival of 
the light from the nucleus, before that from the remoter parts of the 
tail reaches the earth; hence the interval during which the comet 
moves on in its course, is very much augmented, and the result 
is a corresponding increase of curvature in the tail. 
Again, there has sometimes been observed a general obliquity 
of the tail to the prolongation of the radius vector of the comet, 
when near its perihelion, amounting to some degrees. When we 
consider the great difficulty of obtaining a correct measu of 
this deviation, arising from the peculiar circumstances undet 
which the comet is often seen when near its perihelion, this difli- 
culty also vanishes. In the first place, when these angles have 
been measured, the unerring laws of perspective seem to have 
been wholly disregarded. The unequal effect of refraction nA 
upon the nucleus and the extremity of the tail whennear the ho- 
rizon and oblique to it, an effect of no small consequence, has 
been subject to similar neglect. In the second place, very many 
of the visible comets on which observations have: been made 
when near their perihelia, have been visible only when near the 
horizon, from the very fact of their proximity to the sun, and like 
other heavenly bodies, have been occasionally subjected to dis- 
tortion beyond the ordinary effect of refraction, rendering accU- 
rate measurement altogether impracticable. Indeed, the whole. 
train of observations on the tails of comets, seems to have beet 
made with little or no reference to the ordinary influence of the 
oo Te ae aie ee os OO Oe a 
* See this Journal, Vol. x1x, No. 2. 
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