of a Transit Circle . 143 
one’s ease, and thereby very greatly indeed to the accuracy 
and certainty of the observation. 
With this intent, since the microscopes are, the one above, 
I ordered that to be marked A ; the other below, B ; consi- 
dering that the numbers deduced from them could never be 
mistaken, if one got into the habit of examining A first, and 
noting that down, and then examining and setting B under 
it ; which, if all things are true, ought to be the complement 
to 90 degrees. 
To make the reading pleasant, I ordered the micrometer- 
screw in each to be placed on the right hand ; and considered 
the moveable wire as always to be kept to the right hand of the 
other. This will of course, in all cases, measure the distance 
of the fixed wire from the nearest dot apparently on the right, 
(or, since the microscopes invert, the nearest dot really to the 
left) which will be either the degree itself on that hand, or 
some multiple of ten minutes from it. 
That the numbering of the degrees might coincide with 
this idea, I considered, that the figures should be made to ap- 
pear erect in the microscopes, in every position of the tele- 
scope (which they might be whenever it does not point be- 
low the horizon) and that they should be reckoned back- 
wards. To effect this, they ought to be reckoned backwards 
in themselves, but to stand the contrary way, or inverted 
in reality. This would be different in the two microscopes, in 
respect of the centre of the circle ; but that could create no 
difficulty. For, since the two quadrants nearest to the object- 
end of the telescope, would always be those coming under 
the examination of microscope A ; and the two nearest to the 
eye-end, those to be observed at microscope B ; they might be 
U a 
