he stars
which it represents, numbering on the average some 300 or 400; so that for
instance at Oxford the total number of stars measured on the twelve
hundred plates is nearly half a million. These are not all separate stars;
for the sky is represented twice over, and there is also the slight
overlap of contiguous plates; but the number of actual separate stars
measured at this one observatory is not far short of a quarter of a
million, and it has taken nearly ten years to make the measurements, with
the help of three or four measurers trained for the purpose. To render the
measures easy, a network or reseau of cross lines is photographed on each
plate by artificial light after it has been exposed to the stars, so that
on development these cross lines and the stars both appear. We can see at
a glance the approximate position of a star by counting the number of the
space from left to right and from top to bottom in which it occurs; and we
can also estimate the fraction of a space in addition to the whole number;
but it is necessary for astronomical purposes to estimate this fraction
with the greatest exactness. The whole numbers are already given with
great exactness by the careful ruling of the cross lines, which can be
spaced with extraordinary perfection. To measure the fraction, we place
the plate under a microscope in the eye-piece of which there is a finally
divided cross scale; the centre of the cross is placed over a star image,
and then it is noted where the lines of the reseau cut the cross scale. In
this way the position of the image of a star is read off with accuracy,
and after a little practice with considerable rapidity. It has been found
at Oxford that under favourable conditions the places of nearly 200 stars
per hour can be recorded in this way by a single measurer, if he has some
one to write down for him the numbers he calls out. This is only one form
of measuring apparatus; there are others in which, instead of a scale in
the eye-piece, micrometer screws are used to measure the fractions; but
the general principle in all these instruments is much the same, and the
rate of work is not very different; while to the minor advantages and
disadvantages of the different types there seems no need here to refer.
One particular point, however, is worth noting. After a plate has been
measured, it is turned round completely, so that left is now right, and
top is now bottom, and the measurements are repeated. This