, 1882-12-20

moment and declare that that, and no other
is the true instant Of "contact". Every
precaution, however, is being taken to
make the observations as good as the ma
ture of the case allows. The telescopes to
be used are all nearly Of the same size
five or sic inches in diameter the observ.
ers are on their guard against illusions;
most Of them have been practiced on oar
tifcial %transits y mud many OF them saw
tue transit Of 1874; it may fairly be pre-
sumed, therefore, that some advance will
have been made since then. As 10 the
heIiometric method, its use will be pref
ty much coufned to the German parties
who will observe in America at Hart.
ford, Conn, and Arker, N. G, and in the
Southern Hemisphere at Bahia Blanca
(Buenos Ayres) and the Straits of Magel-
Ian. The Yale College Observatory will
make observations Of the same sort with
the magnificent helIometer it has Just re-
ceived from RepsoId-the only one in
the country. The Photographic method,
as tried in 1874, proved for the most pare
q failure, and the international Com
mission, which met at Paris 10 the spring
TO arrange for the observations of the
transit, declined to recommend its fur.
Their use. At that time, however, the
measures of the American photographs
had not been published. They, and a
series of photographs made by q Russian
party at Port Possiet, seem to be an
exception to the general failure. Most of
the photographs were made by attaching
q camera with an enIargiDg lens to a
comparatively small telescope, III this
way very handsome pictures can be OBJ
taiped, but there will be more or less
distortion, and it is very difficult to get
the data tor calculating its precise amount
aud effect. 1t is still more difficult to
ascertain the precise scale of pictures
taken in this Manner e, the number
of seconds OF arc corresponding to each inch
on the plate. 1t is essential that on o 4
inch picture of the sun, the distance and
direction of the centre of the planet from
that of the sun should be measurable
with an error not much exceeding
10000 of an inch, and the scale must
be ascertained with corresponding accu.
racy. In order to secure this degree of
precision, the American astronomers dis
CdIuCu tuO Culhl6lHS ICuS, anu uSeu leIC-
scopes forty feet long, which give directly
pictures..of the requisite size. For obvious
reasons, the telescope is placed horizon
tally and exactly north and south, and
the sunlight is directed into the object
glass by a fat mirror, driven by clock
work. A delicate plumb line, hanging
close in front of the plate and photo.
Praphed upon every picture, determines
what is called it's COMBINATIONS" The
scale is got with extreme precision by a
careful measurement of the distance be-
tween the Object glass and the sensitive
plate. The only ticklish point in the
whole apparatus relates to the absolute
Aatuess of the mirror but the skill of the
Clarks seems fairly to have conquered this
difficulty. At any rate, the results deduced
from the American photographs 10 1874
appear to be at least as self-consistent and
authoritative as those obtained by either
of the other methods. American astrOno-
mers are, therefore, very anxious that
photography shonld have another trial this
winter, and full provision hos been made
for this object.