From the ‘Astronomische Nachrichten’, No. 75 (1826), pp 35-38
(translated by Chris Plicht, annotations in () by CP)
Appendix to the Article on the Great Refractor
It was not intended to print the article in No. 74 and it was not written
for astronomers; therefore it contains some unnecessary details. A thorough
description should have several drawings of the various parts with it. I had
made a perspective drawing of the complete instrument; this drawing, which
shows the side where the clockwork is located, is attached to the article.
When the instrument was shipped for Dorpat, the micrometers for it were
not finished; those will be shipped later and reach their destination before
the final set-up of the refractor.
Of the filar micrometer dedicated for this instrument, both wires, independently
of each other, may be moved by a screw; in part to set it
where it is needed, in part to make repetitive measurements when observing,
which is much easier done with the refractor moved by the clockwork rather
than with the usual setup. In the same sense the eyepiece is movable, so that
both wires are at equal distance from the centre of the field of view, in
this case the discernability is the same for both. The part of the micrometer
holding the wires, contains, with the necessary correction screws etc., two
Verniers in opposite locations which move on a divided circle that is used
to measure the position angle. The verniers give directly one minute. The
micrometer may be set in relation to the position circle, not
only by hand but also smoothly with a screw. The wires may be individually,
in the dark field of view, illuminated. Since the position
circle, in relation to the optical axis, must be fixed, but the micrometer
with the wires and lighting equipment must be rotatable, a special setup is
necessary which is different from the one I used earlier for the lamp filar
micrometer without position circle. Also the field of view may be illuminated.
This micrometer has four different eyepieces.
In addition to the filar micrometer, the refractor has: an illuminated circle
micrometer with four eyepieces;
An illuminated net micrometer with three eyepieces; finally four ring micrometer
of which two have double rings.
Since only in the case, when the (optical) axis of the objective is coincident
with the (optical) axis of the eyepiece the definition is best, and a deviation
in this relation is a greater disadvantage with larger objectives than with
smaller ones, a special instrument will be made for this refractor with which
the deviation may be controlled and corrected.