certainty hardly inferior to that of ocular demonstration." Airy discussed
the matter with Professor Challis (who, it will be remembered, had
originally written to him on behalf of Adams), suggesting that he should
immediately commence a search for the supposed planet at Cambridge. It may
be asked why Airy did not commence this search himself at Greenwich, and
the answer is that he had no telescope which he regarded as large enough
for the purpose. The Royal Observatory at Greenwich has always been, and
is now, better equipped in some respects than any other observatory, as
might be expected from its deservedly great reputation; but to possess the
largest existing telescope has never been one of its ambitions. The
instruments in which it takes most pride are remarkable for their
steadiness and accuracy rather than for their size; and at that time the
best telescope possessed by the observatory was not, in Airy's opinion,
large enough to detect the planet with certainty. In this opinion we now
know that he was mistaken; but, again, we must not judge his conduct
before the event in the light of what we have since discovered. It may be
recalled here that it was not until Le Verrier's third paper, published on
August 31, that he (Le Verrier) emphatically pointed out that the new
planet might be of such a size as to have a sensible disc; and it was this
remark which led immediately to its discovery. Until this was so
decisively stated, it must have seemed exceptionally improbable; for we
saw in the last chapter how diligently the Zodiac had been swept in the
search for minor planets,--how, for instance, Hencke had searched for
fifteen years without success; and it might fairly be considered that if
there were a fairly bright object (such as Neptune has since been found to
be) it would have been discovered earlier. Hence Airy not unreasonably
considered it necessary to spread his net for very small objects. On July
9 he wrote to Professor Challis as follows:--

No. 15.--G. B. AIRY _to_ PROFESSOR CHALLIS.

"THE DEANERY, ELY, _1846, July 9_.

"You know that I attach importance to the examination of that part of
the heavens in which there is ... reason for suspecting the existence
of a planet exterior to _Uranus_. I have thought about the way of
making such examination, but I am convinced that (for various
reasons, of declination, latitude of place, feebleness of light, and
regularity o