Attempt to apply instrumental measurement to the Zodiacal Light

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                                generally between them and the Northern (or point of culmina
tion for the Southern Hemisphere). Were the spectator to face the 
Northern point exactly, then the equator would be represented 
by a straight line, parallel with the horizon, and elevated 
above it at an angle equal to the co-latitude  of the place. 
And according to the degree to which the spectator turns 
round from the East or West points towards the North, the incli=
=nation of the Equatoreal line will vary, from the angle of the 
latitude of the place, to perfect horizontality. 
This varied inclination of the equator has been 
strictly attended to in all the accompanying cases; but it has 
been found advisable for simplicity in practise, to represent the 
declination circles also by straight lines, for there is hardly any 
sensible difference caused thereby in the central region of the 
picture, where all the important part of the subject to be deline=
=ated, lies; and although the configuration of stars near the borders 
might not be such as would exactly appear to the eye of a 
spectator, or as they should be represented by the usual rules 
of perspective, still the amount of discordance is so extremely 
small, that the nicest eye would hardly perceive it, and, 
what is far more important than that, a  representation per=
=fectly similar in the minutest particular to that formed 
in the retina, still the particular projection of the sphere which 
<u>was<\u> employed there, being inserted in the margin, gives just 
as good and rather simpler means than would have been 
available on the other plan for the identification of the stars. 
The application of instrumental measurement, herein=
=before described, to determine the phenomena of the Zodiacal 
Light, is believed to be new; and the observations so obtained, 
seem to show very <s>readily<\s> decidedly, especially those of October, 
1844, that numerical measures of the place of the apex of the 
Light may be easily obtained, - with a probable error of 
not more than two degrees: so that vague estimations & notes 
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Manuscript details

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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