Bristol news. [volume] (Bristol, Va. & Tenn.) 1867-189?, 1873-12-23

/ can then be received by an eye-glass
precisely IS the image iq the focus of q
| lens, already described. But to see
| this image an observer must I lace his
head between the object and the mir
rOr, and hence cut off a large portion
of light from the mirror. To avoid
| this difficulty, Sir Isaac Newton.
| placed & small mirror near the focus.
which reflected the rays in lateral
| direction, so that the observer could
see the image by rejecting through
| the side of a tube. Sir VIM. Herschcl
| tipped his mirror slightly 10 one side,
so that the image was formed near the
| side of the tube. But both these plans
are subject to the great Inconvenience
| that the observer must Ststand ut the top
| Of the telescope. and hence, in the lar
ger telescopes, must lie forty or fifty
feet in the air.
In the Gregorian telescope the diff
ficulty is avoided by placing a second
| small cOncave mirror near the focus OF
| the large one, SO as to throw the light
| back against the centre of the latter.
| Then hole is cut through it in which
a second image is formed, which the
! observer views with an enlarging lens.
| But in this construction large amount
! is lost by two retiectOrs. SO that less
than half OF it can reach the eye of the
| observer.
REFRACTORS VS.. REFLECTIONS.
| The lecturer then entered upon a
comparison Of the reflecting and Ie-
| ftacting Telescopes. Most of the great
| telescopes which have been celebrated
as those of HerSchel. ROsse, and Las
! sci, belong to the class of retectors.
| In practice. however, these great in
/ Struments have never been found ef-
! fective In proportion 10 their size. The
| largest ever made is that of Lord Rose,
| and the lecturer expressed himself as
| continually pestered with the question
| how the new telescope lately mounted
at the observatory compared with that
of Lord Rose: He would reply To-
| this Fist, that owing TO the entirely
| different construction of the two i'll
struments, it was hardly possible 10
| institute q comparison between them
and, second, that Lord Roses tele
scope had not proved so effective as
| the mnch smaller refiectors of Mr. Las
sci. The latter gentleman had made
| refectOrs of two feet and four feet dia>
meter, but it did not appear that he
could do much more with the latter
than with the former. As the new
Washington telescope is equivalent to
theoretically perfect reflector of three
feet diameter, he supposed it would be
about as effective rs any of those great
refiectors.
Tie Professor then briefly described
some of the principal refracting tele
scopes. A PAIR of fifteen inches aper-
ture, made about the year 1910, from
the observatories of Cambridge and
Pui Rowa 10y MeIz and Aahln, Of Mu
nich, were considered wonders at the
time For twenty years they made
the largest which nave been brought
INTO successful Operations. But IN 1860
Mr. Alvan Clark, o. Cambridge, Mass
made one of eighteen inches which
was sold to an observatory in Chicago.
This was soon exceeded by an English
firm, who made one of no less than
twenty-five inchs aperature, for pri-
vate gentleman in England. Deter
mined to exceed every one, Mr.. Clark
has now mdde ofie of twenty-siX in
ches, for the observatory here, which
is therefore the largest rejecting tele-
scope ever successfully completed.