He immediately began with his usual energy to organise the systematic
conduct of the business of the National Observatory. To realise one
of the main characteristics of Airy's great work at Greenwich, it is
necessary to explain a point that might not perhaps be understood
without a little explanation by those who have no practical
experience in an observatory. In the work of an establishment such
as Greenwich, an observation almost always consists of a measurement
of some kind. The observer may, for instance, be making a
measurement of the time at which a star passes across a spider line
stretched through the field of view; on another occasion his object
may be the measurement of an angle which is read off by examining
through a microscope the lines of division on a graduated circle when
the telescope is so pointed that the star is placed on a certain mark
in the field of view. In either case the immediate result of the
astronomical observation is a purely numerical one, but it rarely
happens, indeed we may say it never happens, that the immediate
numerical result which the observation gives expresses directly the
quantity which we are really seeking for. No doubt the observation
has been so designed that the quantity we want to find can be
obtained from the figures which the measurement gives, but the object
sought is not those figures, for there are always a multitude of
other influences by which those figures are affected.
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