Attempt to apply instrumental measurement to the Zodiacal Light

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                                at any other time of the year. But what the cause of it in nature 
should be, I cannot yet imagine, but leave it to further inquiry". 
Here then is a clear and simple account of one phase of the 
phenomenon, marking it as something unusual, as different 
from ordinary twilight, as constant in that anomalous difference 
and therefore being well worthy of being carefully inquired into. 
In his travels in Persia in 1668 (Edit. de Langles, T.IV, p 
326, & TX, p97.) Chardin mentions having seen the tail of the great 
comet of that year above the Western horizon after sunset; the head 
being visible only in the Southern Hemisphere. Cassini and Mairan 
writing some years after, under the influence of the then new 
discovery of the Zodiacal Light, asserted that it must have been 
this which Chardin saw; and he is even made out by Delambre 
to have been the original discoverer of it. The Comet of 1668 having however 
appeared again in 1843 (that is, they are supposed with the 
greatest probability to be identical; and if not identical, still 
they are at least specimens of the comet genus), has given 
us the opportunity of determining whether Chardin's description 
applies to the Zodiacal Light or to the Comet, which though so 
very unlike each other, were not only confounded at the former 
apparition, but at the latter also; when the tail, as before, was 
the only part visible in the Northern Hemisphere. The slightest 
glance at the accompanying drawings of the two objects however 
will probably convince every one, that Chardin's Persian expression 
"nirzouk", or in French "petite lance",  which was applied by the 
Persians to the phenomenon they saw, could only be considered as at 
all suitable in the case of the Comet's tail. 
In 1683 the subject was taken up by Dominic Cassini, 
& to him belongs the merit of first scientifically investigating 
the laws of the phenomenon, determining its cosmical nature, & 
giving it the appropriate name of the Zodiacal Light. His series 
of observations, extending over nearly six years, is still unrivalled; 
and if he is not correct in all his conclusions, it is chiefly because 
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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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