verlap of contiguous
plates. And more than this, it was decided that the whole sky should be
completely covered _twice over_. It was conceivable that a question might
arise whether an apparent star image on a plate was, on the one hand, a
dust speck, or, on the other hand, a planet, or perhaps a variable or new
star. By taking two different plates at slightly different times,
questions of this kind could be settled; and to make the check more
independent it was decided that the plates should not be exactly repeated
on the same portion of sky, but that in the second series the centre of a
plate should occupy the point assigned to the corner of a plate in the
first series.

[Sidenote: Times of exposure.]

Then there came the important question of time of exposure, which involved
a long debate between those who desired the most modest programme possible
consistent with efficiency, and those enthusiasts who were anxious to
strain the programme to the utmost limits attainable. Ultimately it was
resolved to take two series of plates; one series of long exposure which
was set in the first instance at 10 minutes, then became 15, then 30, then
40, and has by some enterprising observers been extended to 1-1/2 hours;
the other a series of short exposures which have been generally fixed at
6 minutes. Thus instead of covering the sky twice, it was decided to cover
it in all four times, and the number of plates assigned to each
observatory in the above list must be regarded as doubled by this new
decision. And further still, on the series of short-exposure plates it was
decided to add to the exposure of six minutes another one of three
minutes, having slightly shifted the telescope between the two so that
they should not be superimposed; and later still, a third exposure of
twenty seconds was added to these. It would take too long to explain here
the reasons for these details, but it will be clear that the general
result of the discussion was to extend the original programme
considerably, and render the work even more laborious than it had appeared
at the outset.

[Sidenote: Measurement of plates.]

[Sidenote: The reseau.]

[Sidenote: The microscope.]

[Sidenote: Reversal of plates.]

[Sidenote: Personal equation.]

When all these plates have been taken, the work is by no means finished;
indeed, it is only just commencing. There remains the task of measuring
accurately on each of the short-exposure plates the positions of t