Attempt to apply instrumental measurement to the Zodiacal Light

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                                "the light, probably on account of the faintness with which it 
appears to us in this part of the world. Your are certainly right in 
ascribing the rapid variations in the light of celestial objects, 
which you have perceived in the climate of the tropics, to changes 
taking place in our atmosphere, and especially in its higher 
regions. This shows itself in a most striking manner in the 
tails of great comets. Often, and particularly in the clearest 
weather, pulsations in the tails of comets are seen to commence 
from the head or nucleus as the lowest part, and to run in 
one or two seconds through the whole extent of the tail, which 
in consequence appears to lengthen several degrees, and contract 
again. That these undulations, which engaged the attention 
of Robert Hooke, and in later times of Schr[o umlaut]ter amd Chladni, 
<u>do not take place in the cometary tails themselves<\u>, but are 
produced in our atmosphere, appears evident if we reflect 
that the several particles of these cometary tails (which are many 
millions of miles in length) are at <u>very different distances<\u> 
from us, and that the light from them can only reach 
our eyes at intervals of time which differ several minutes 
from each other. I will not attempt to decide whether what 
you saw on the banks of the Orinoco, not at intervals of seconds, 
but of minutes, were actual coruscations of the Zodiacal light, 
or whether they belonged solely to the upper strata of our atmos=
=phere. Nor can I explain the remarkable lightness of entire 
nights, or the anomalous increase and prolongation of twilight 
in the year 1831, particularly if, as it has been said, the <u>lightest 
part<\u> of these singular twilights did not coincide with the 
place of the sun below the horizon." 
Sir John Herschel's views published only 5 years ago, 
were called forth by the tail of the great comet of 1843, having 
been by some so pertinaciously mistaken  for the Zodiacal light. 
"The Zodiacal Light", said he, "as its name imports, invariably 
appears in the zodiac, or, to speak more precisely, in the plane 
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Charles Piazzi Smyth
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Attempt to apply instrumental measurement to the Zodiacal Light , 1840. From The Royal Society, AP/30/18



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