STANDARD - Survey Instrument Antique Center!

13
has sufficiently developed to impair the transparency of these lenses. J3ut whenever
it is found that a film has settled between them, then it is best, if the distance
is not too great, to send the whole, instrument to its maker, and if this is not feasible,
then the telescope, at least, well and soft packed in a box, should be sent.
Cemented Object-glasses. To prevent the settling of a film between the
lenses composing an object-glass, and to avoid disturbing reflections of light from
their inner surfaces, such films and reflections imparting to an object viewed
through a telescope the hazy appearance noticeable in high-powered telescopes,
we now, since 1889, cement these lenses together, so as to form one lens only. The
lenses so treated are more efficacious in many respects than when separated by
three thin pieces of tin foil, as has been the custom of nearly all instrument
makers up to date.
The cement, however, needs some five or six months to harden, and until it has
hardened sufficiently, an exposure to a cold atmosphere causing a greater contraction
of the metal cell than the glass, the lenses are very apt to warp, which may
lead to a distortion of an object, when viewed through such an objective.
The proper treatment of an object-glass freshly cemented is to keep the instrument,
when not in use, in a room having a mean temperature of about 68 F., or
slightly above. The same treatment should be followed if it is found that the
image formed of an object is slightly distorted ; only in this case the temperature
in which it is kept over night should be raised to about 75 or 80 F. This treatment
applies only to normally mounted objectives. If they are too tightly fitted
the lenses cannot be restored to their original efficacy without being attended to by
a maker.
Object-glasses that are cemented are very apt to show some specks, or, with ill
usage, cracks in the cement, but, unless the specks are very numerous, so as to
cover almost the whole area of the object-glass, the opacity caused by them does
not sensibly affect the efficacy of the telescope, and there core need not disturb the
mind. Our experience is that the usefulness of an instrument is greatly enhanced
when these lenses are cemented together, and that a few specks that may appear
after an exposure from a sudden change from hot to a very cold atmosphere, or vice
versa, are a lesser evil, as compared with the ill effects produced by a film that in
time will settle between these lenses if separated by pieces of tin foil, or even when
brought in direct contact with each other, as such a film will have much the same
effect as a fog, in preventing vision.
When, after carefully cleaning the lenses of a telescope, the object-glass of which has
its lenses separated by pieces of tin foil, it is found that the image is not as clear as
originally, it is a sure sign that there is a film between its lenses, and that it has been
exposed to a damp or impure atmosphere, either by injudicious use in the field, or by
being left too long a time in the packing box, in which it is protected by cushions of
paper or shavings, both of which attract moisture, or by storing it away in its box in
such an improper place as a basement or cellar. Such film being noticed, it will then
be well to send the object-glass, or much better, the telescope, or, best, if the distance is
not too great, the whole instrument, to the maker, in order that the lenses may be
cleaned by him, and, if deemed advisable, be cemented. The slight expense incurred
of a few dollars will be more than justified by the advantage gained.
When the object-glass, or telescope is returned after the cleaning or cementing of
its lenses, the cross-wire, spirit level, and vertical arc adjustments of the instrument
will require a thorough verification before it should be used. In case the whole instrument
has been sent to the maker, these adjustments are attended to by him. If the object-glass
has been cemented, the telescope should be watched for a year to see that there
is no distortion of the image. If there is a distortion, it will indicate that the objectglass
has been too tightly fitted, of which fact we should be informed, as also whether
after cementing the object-glass the instrument retains its cross-wire adjustment the
same as before the cementing took place. If the cross-wire adjustments have to be
more frequently made than before the lenses were cemented, it indicates that the objectglass
is not tightly fitted to its cell and if such is the ; case it should .be sent to us to be
more tightly fitted, after a lapse of about ten or twelve months, when the cement will
have sufficiently hardened to allow of a tighter fit of the object-glass in its cell.
In telescopes of very high power it is of as great importance to keep the lenses of the
eye-piece free from grit and films as of the object-glass. Therefore, whenever the telescope
does not appear to be clear, the lenses of the eye-piece need most careful cleaning
(if necessary, every four weeks). The cleaning must be done by first wiping gently
with a clean piece of old linen barely moistened with alcohol and then wiping dry, using
a clean part of the linen on every surface of the lenses. (Please read the various
articles on this point on pages 10, 11 ,and 31, of our handbook and catalogue.) To remove
the eye-piece, unscrew the German-silver screw at the eye-end of the telescope.
Of course, after cleaning, every lens must be put back in its tube precisely as marked,
and then the outer bearings of the eye-piece in the main tube must be greased with
tallow before the German-silver screw is restored to its place.