It sometimes happens that the films do not last so long as they are
expected to do, that is, they sometimes lose their splendid lustre
sooner than they should do. When it is remembered that though the
process of silvering is an easy operation to perform, yet it is a
delicate chemical one, in which good results are best obtained when
several good conditions all meet together, this is not to be
wondered at, but it is certain the instructions given above will
reduce the chances of failure to a minimum, and imperfect results
will be rare. The silver usually lasts a long time. I know silver
films that have been in use for ten years.
Do not suppose that a long and badly worn surface will fail to show
the detail and colours of the planets, the fullest detail also of
the moon or the stars. It is surprising how long a silvered surface
will continue to do its full work while its appearance is much
deteriorated; picking up faint points of light that are near the
very limit of its aperture and power, seem alone to require the
perfect and fresh film. I know silver films that have been in use
for seven years and have not been ever re-polished.
That the Silvered Glass Reflecting Telescope is giving satisfaction
and is capable of performing the best work of a telescope—being
durable, convenient, and perfectly efficient—may be gathered from
the selection of a very large number of satisfactory and most
gratifying letters received by the writer; and it has been most
encouraging to find in all cases the observer is most eager to
express his entire satisfaction, and especially so where so many
have been prejudiced, in the absence of experience, with feelings of
doubt as to their _complete_ efficiency, and many, again, having
used fine refractors.
It will be seen, too, that many have commenced with a small sized
reflector, and gone on increasing their optical power until they
have obtained a large and powerful instrument, imposing indeed as
compared with what was considered a powerful instrument less than
half a century ago, and at a fraction of its cost.
The gratifying success which I have achieved, attested by the
universal satisfaction given by the instruments supplied by me, is
the result of unwearying labour and untiring patience; every
speculum, large or small, being figured with my own hands.
I have every facility for setting up instruments, and for testing,
working, and regulating them in every part on celestial objects. The
optical parts are, therefore, not merely tested separately and
subsequently mechanically adjusted, but each and every instrument is
put together and optically and mechanically tested, as a whole,
before it is permitted to leave the workshop.
Every speculum is most carefully figured, and confidently guaranteed
perfect _to the extreme edge_.
The employment of “stops” of any kind is quite unnecessary, except
on bad nights, when the aperture must of course be suited to the
degree of steadiness of the air. For, whether the instrument be a
reflector or a refractor, the aperture and power used must of
necessity be limited by the atmospheric conditions under which
observations are made.
It should, however, be remembered that the reflector having not only
a much larger aperture than a refractor of same focal length, and,
being open to the influence of the external air, is not only
affected by a relatively larger column of air, but is affected in a
different way. It frequently happens that reflectors of 6 in. to 18
in. aperture, are of same focal length as 5 or 6 inch refractors.
These considerations will explain the occasional use of stops, and
the expediency of having them ready to use when they can be of
temporary advantage. There are nights in our climate when a 6 inch
aperture may be the largest that can be used with satisfaction or
advantage; indeed it used to be accepted as an incontrovertible fact
that an aperture of G inches was the largest that could be used on
_average_ nights in our country. But there are also not a few nights
when large apertures, bring great gain to their fortunate
possessors.
It is obvious that, _mutatis mutandis_, these conditions must affect
refractors as well as reflectors, with perhaps this difference, that
in the case of a reflector the rays pass down the tube and infringe
on the speculum as a column of parallel rays; consequently all the
external rays of the column travel near the surface of the tube for
its entire length. This being so, it is obvious that until the
internal and external air and the metal tube have time to equalize
in temperature, it will be advantageous, in all work requiring high
powers, such as the examination of difficult double stars, to “stop
off” for a time the _extreme edge_ by a small diaphragm.
This will occur especially on frosty nights after a warm fine day.
When the telescope is in the open air, ice will not infrequently
form on the top of the tube, and the effect will be at once obvious
in the irregularity of a star image at the edge corresponding to the
top of the tube. The use of a small diaphragm will at once obviate
this defect and give perfect images. On such nights a wooden tube is
preferable to a metal one. It has been frequently my experience,
when finally testing an instrument, that when I have been
dissatisfied with its performance, and in order to trace the cause
of the apparent defect, have removed the mirrors to my wooden
testing tube, I have found every defect removed, and imperfect
replaced by perfect images. The explanation is simple. Not only is
the wooden tube much larger in diameter than the specula, but wood
is of itself less sensible than metal to differences of temperature.
To make the tubes some 2 or 3 inches larger in diameter than the
speculum would cure this evil, but it would involve various
inconveniences and expenses in structure, which would more than
outweigh the advantage of overcoming a hindrance which is after all
only temporary and occasional.
The experiment of perforating or ventilating tubes has now been
thoroughly and exhaustively tried, with the result that there is a
fairly general consensus of experienced opinion against their use.
The advantages are merely theoretical; the disadvantages are grave.
Among them are the admission of cross reflections in every
direction, the admission of dust, imperfect protection of the
mirrors, and a decided loss of strength and rigidity to the tube.
CATALOGUE OF Silvered Glass Reflecting Telescopes, &c.,