Attempt to apply instrumental measurement to the Zodiacal Light

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                                too great an influence on the phenomenon to be observed, to allow 
of strict comparison being made between the observations at the 
various places. But there was at least the possibility of being able 
to determine a good method of making the observations, by endea=
=vouring to reduce to practise some plan by which the results 
should be expressed more in numbers than has generally been the 
case; and of ascertaining at least the degree of accuracy with which 
observations of place, i.e. of AR[?] & D could be made; in more or less 
favourable localities or seasons. 
I proposed to myself thereforeto endeavour to determine 
each night the AR[?] &Dec<sup>n<\sup>. of the apex of the light; though the 
only method which was then available, viz. observing the parti=
=cular stars amongst which the point was situated, was only 
of use when there were large stars close by; since even if there 
had been star maps to refer to in the desert, to identify the 
smaller ones, the phenomenon to be observed was not one that 
would bear close and direct investigation. It was only after 
having closed the eyes for some little time, or having turned 
them to some dark part of the sky or earth, - that on suddenly 
directing them to the region of the Zodiacal light, but not 
directly to the middle of it, it was seen of a well defined figure: 
for by looking straight at it, and still more by coming 
into contact with any artificial light, the situation of the 
apex appeared to vary many degrees, or could not be decided 
on at all. 
At length therefore, in 1844, I made a little wooden 
instrument with equatorial motion, plain sights, & roughly 
divided circles; which being placed in position in some spot 
free from any artificial light, readily gave the means of 
determining the object sought. For by the sort of side-glance above described, a good 
notion of the position of the apex being obtained, the plain 
sights were immediately pointed to the spot, the circles read 
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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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