eflector might be
described as an instrument with a temper; sometimes it gives excellent
results, but at others _something_ seems to be wrong, though the worried
observer does not exactly know what. Long experience and patience are
requisite to humour the instrument and get the best results from it, and
it was felt that this uncertainty was sufficient to disqualify the
instrument for the serious piece of routine work contemplated in mapping
the heavens. Accordingly the handier and more amiable instrument with
which the brothers Henry had done such good work was selected as the
pattern to be adopted.

[Sidenote: Doublet would have been better.]

It is curious that at the Conference of 1887 nothing at all was said about
the type of instrument first mentioned (the "doublet lens"), although a
letter was written in its favour by Professor Pickering of Harvard College
Observatory. Since that time we have learnt much of its advantages, and it
is probable that if the Conference were to meet now they might arrive at a
different decision; but at that time they were, to put it briefly,
somewhat afraid of an instrument which seemed to promise, if anything, too
well, especially in one respect. With the reflector and the refractor it
had been found that the field of good images was strictly limited. The
Henrys' telescope would not photograph an area of the sky greater in
extent than 2 deg. in diameter at any one time, and the reflector was more
limited still; within this area the images of the stars were good, and it
had been found that their places were accurately represented. Now the
"doublet" seemed to be able to show much larger areas than this with
accuracy, but no one had been able to test the accuracy to see whether it
was sufficient for astronomical purposes; and although no such feeling was
openly expressed or is on record, I think there is no doubt that a feeling
existed of general mistrust of an instrument which seemed to offer such
specious promises. Whatever the reason, its claims were passed over in
silence at the Conference, and the safer line (as it was then thought) of
adopting as the type the Henrys' instrument, was taken.

[Sidenote: The eighteen observatories.]

This was perhaps the most important question settled at the Conference,
and the answers to many of the others naturally followed. The size of the
plates, for instance, was settled automatically. The question down to what
degree of faintness should stars