Worcester daily spy. [volume] (Worcester [Mass.]) 1850-1888, 1871-08-07

just at the end of the Brookline bridge, in
Cambridge, stands understanding brick build
ing that most persons would not consider worthy
OF a second glance if indeed it attracted their
attention, and near this building is another sur
mounted by dome. These form the establish
ment of Alvan Clark, who has worldwide
reputation as u builder of telescopes, and also
as an amateur astronomer.
Mr.. Clark was originally miniature painter,
and quite a successful one, as many paintings
still iu his possession prove. Some years ago
his attention was accidentally called to the sub
ject of telescope making, from description
given by one of his sons of the small telescope
at Amherst college, After hearing this, MI.
Clark remarked that he could make telescope,
and accordingly went 10 work and made ono,
His first instrument proved so successful that
he was encouraged to perSevere. One of his
friends remarked TO him about this time, MI.
Clark, if you wish to know how to make tele-
scopes, you will have to go to the place where
they are made" But Mr., Clark thought some
things might as well be studied out as learned
from another, and so persevered, and was soon
enabled to retaliate upon his friend by telling
him that he had learned how to make telescopes
independent of instructors.
His telescopes are the kind known as reiract-
ing. In these the principal parts are the field or
obect-glasses, or Objectives, as they are most
frequently called, and the eye-piece The prin-
cpa use of the detectives is to collect the rays
Of light coming from the object and TO form an
image at the focus of the lens. This image is
afterward magnifed by the eye-piece.
On the perfection of this local image depend
to great extent the power and capabilities Of
the instrument. 1t must be coorless, or at
least have no fringes on the edges; it must
also be sharp and well defined, and have large
amount of light thrown on it, The first two of
these conditions may be fulfilled with a glass of
a comparatively small diameter, but in order to
accomplish the third the diameter must be in
creased to as great an extent as possible.
Mr.. ClaTk's first telescopes were distinguished
for their freedom from chrOmatic and spherical
alteration, as the first two errors are called and
latterly he has been devoting his time more es-
pecially to increasing the size of the glasses. In
order to correct the chromatic alteration the
objective is made of two disks of glass. The
first of these, or the convex lens, is made of
crown glass. This alone would form an image
at the focus, but the image would be fringed
with the colors of the spectrum. In order to
prevent this eoneave-conveX lens, made of flint
glass, is placed directly behind the first glass;
this re-composes the white light which the
first lens has decomposed, and forms an image
at the focus almost perfectly eolorless. In
order to make it perfectly colorless we should
have to use more than two lenses.
There still remains the spherical alteration to
be corrected. This is owing to several causes.
In the first place, it may be that the glass is a
little more dense on one side than the other;
this will serve to distort the image somewhat,
and must be corrected by proper grinding.
Then portion of the glass may polarize light
while another portion does not; this must also
be corrected by grinding.
In order to test the glasses for these errors,
they are placed on carriage and run into a
long tunnel which has small spot of light at
the further end. If now the eye is placed at the
focus OF the lens, sharp, clear image of the
spot will be seen, provided the lens is perfect;
otherwise it will be distorted and colored. 1t
must also, when examined with Nicols prism,
still remain free from color. If this is not the
case, the glass is removed, repolished, and the
test is repeated.
In order to grind the glasses, a coarse emery
is first used, until they have been brought very
nearly to the perfect form required. Then finer
grades of emery are applied, and finally they
are polished with rouge. The process takes a
long time from the first arrival of the rough
glass until it becomes the finished objective.
Almost the whole of the grinding is done by
hand, or laps made of cast iron. These are
used at first naked, and afterward covered with
pitch.
Telescopes pre- generally spoken of as being of
so many inches aperature; and until within a
very few years telescope Of fifteen inches aper-
ture was regarded as enormous. In 1861, Mr..
Clark constructed one of twenty inches aper-
ture for the observatory at Chicago. This, at
the time, was the largest refracting telescope
ever made. Since then one Of twenty-fourinches
aperature has been made in England; and row
the Messrs Clark have orders for two of twenty
five inches aperture; one intended flor the gov
arnment observatory at Washington, the other
- go to Virginia.
These telescopes will be about twenty-five
feet long when finished, the rule being that q
telescope shall be as many feet long as the OBJ
jective is inches in diameter. Any one who
wishes to indulge himself with one of these in
struments as a toy can do so at an expense
of about 100000, and there will most likely be
a further expense of $4,000 in providing 9
place to keep it. These large telescopes are SC
nicely adjusted that person can easily move
them with finger, although they weigh several
tons. Besides making telescopes, Mr.. Brown has
made a speciality of spectroscopes. He has con.
structed some very fine ones; among others
that of Prof Young, to which we have alluded
in the Journal. This instrument has been al
most completely rebuilt, and some improve
ments suggested by experience have been added.
Mr.. Clark has also built new telescope for
nine and one-third inches aperture. This in
strument has a steel tube, made Of plates OF
steel riyeted together, as it has been found that
steel tubes can be constructed so as to weigh
less and yet be sitter than wooden ones.-Bost0n
Journal Of Chemistry.