rty-two pages of foolscap, and the other sixty; and it must
be remembered that the calculations themselves were quite novel at that
time. Of his _skill_ in calculation, apart from his assiduity, we have a
proof in a paper communicated to the Royal Society rather later (1726),
where he determines the longitudes of Lisbon and New York from the
eclipses of Jupiter's satellites, using observations which were not
simultaneous, and had therefore to be corrected by an ingenious process
which Bradley devised expressly for this purpose. And finally, his skill
in the management of instruments is shown by his measuring the diameter of
the planet Venus with a telescope actually 212-1/4 feet in length. It is
difficult for us to realise in these days what this means; even the
longest telescope of modern times does not exceed 100 feet in length, and
it is mounted so conveniently with all the resources of modern
engineering, in the shape of rising floors, &c., that the management of it
is no more difficult than that of a 10-foot telescope. But Bradley had no
engineering appliances beyond a pole to hold up one end of the telescope
and his own clever fingers to work the other; and he managed to point the
unwieldy weapon accurately to the planet, and measure the diameter with an
exactness which would do credit to modern times. A few words of
explanation may be given why such long telescopes were used at all. The
reason lay in the difficulty of getting rid of coloured images, due to the
composite character of white light. Whenever we use a _single_ lens to
form an image, coloured fringes appear. Nowadays we know that by making
two lenses of different kinds of glass and putting them together, we can
practically get rid of these coloured fringes; but this discovery had not
been made in Bradley's time. The only known ways of dealing with the evil
then were to use a reflecting telescope like Newton and Gregory, or if a
lens was used, to make one of very great focal length; and hence the
primary necessity for these very long telescopes. They had another
advantage in producing a large image, or they would probably have given
way to the reflector. This advantage is gradually bringing them back into
use, and perhaps in the eclipse of 1905 we may use a telescope as long as
Bradley's; but we shall not use it as he did in any case. It will be laid
comfortably flat on the ground, and the rays of light reflected into it by
a coelostat.

[Sidenote: Bradl