ividual star. They proceeded patiently with the
work on these lines; but when they came to the region where the Zodiac is
crossed by the Milky Way, and the number of stars in a given area
increases enormously, they found the labour so great as to be practically
prohibitive, and were in doubt how to deal with the difficulty. It was at
this critical moment that these comet photographs, showing the stars so
beautifully, suggested the alternative of mapping the stars
photographically. They immediately set to work with a trial lens, and
obtained such encouraging results that they proceeded themselves to make a
larger lens of the same type; this again was satisfactory, and the idea
naturally arose of extending to the whole heavens the scheme which they
had hitherto intended only for the Zodiac, a mere belt of the heavens. But
this rendered the enterprise too large for a single observatory. It became
necessary to obtain the co-operation of other observatories, and with this
end in view an International Conference was summoned to meet in Paris in
1887 to consider the whole project. There were delegates from, if not all
nations, at any rate a considerable number:--

France 20
British Empire 8
Germany 6
Russia 3
Holland 3
U.S. America 3
Austria 2
Sweden 2
Denmark 2
Belgium 1
Italy 1
Spain 1
Switzerland 1
Portugal 1
Brazil 1
Argentine Republic 1

[Sidenote: Choice of instrument.]

[Sidenote: Expense of "doublet."]

[Sidenote: Advantages of reflector.]

[Sidenote: Refractor chosen.]

The Conference had a number of very important questions to discuss, for
knowledge of the photographic method and its possibilities was at that
time in its infancy. There was, for instance, the question whether all the
instruments need be of the same pattern, and if so what that pattern
should be. The first of these questions was settled in the affirmative, as
we might expect; in the interests of uniformity it was desirable that the
maps should be as nearly similar as possible. The second question was not
so easy; there were at least three different types of instruments which
might be used. First of all, there was the photographic lens, such as is
familiar to all who have used an ordinary camera, consisting of two lenses
with a space between; though sinc