STANDARD - Survey Instrument Antique Center!

81
Transit Solar Attachment.
For running Meridian or other lines by the Sun.
Written for tnis catalogue with special reference to the wants of Public Land Surveyors, for both common and
mineral lands, by J. B. DAVIS, Assistant Professor of Civil Engineering,
University of Michigan.
1. Remarks. The attachment herein referred to is the Davis and Berger
solar screen, prism, and colored shade glass, used for direct solar observation.
These inventions have been devised by the Mr. Berger, of the firm of C. L. Berger
& Sons, and by the writer. They are simply for the purpose of enabling one
to make an observation directly upon the sun's centre. This observation being
secured by readings of the horizontal and vertical circles, is reduced so as to
give the direction of the line of sight of the transit at the instant of the observation.
Thus knowing the direction of the line of sight at a given instant it becomes
simply necessary to turn off the angle which this line of sight makes with the
meridian, to ascertain the position of the meridian. This angle is what is obtained
by reducing the observation, as above mentioned. A brief reference to the history
of these devices will best explain them. It occurred to the writer to see if an image
of the sun could be formed behind the eye-piece of a telescope at the same time
an image of the cross-wires was, and the latter image be made to quarter the
former, by allowing the sun to shine into the object end of the telescope and thence
directly through it. The experiment was made by holding a piece of white paper
behind the eye-piece, and adjusting the focus of the eye-piece and object glass.
The very first trial was readily successful. The next thing was to see if the position
of the instrument could be located by this means as near as the circles would
read. By the same simple means it was soon found that a motion given to the
telescope by either tangent screw might be so slight that the eye could not detect
it upon the circles, but evidence of it would be apparent in the position of the images
with reference to each other. This fact at once settled the question of whether
this would be a sufficiently delicate means of observation. It showed that the
observations would be closer than the circles would read. After some trials and
some months rest these facts were brought to the notice of others, and finally were
submitted to Mr. Berger for his opinion. He made a screen \vhich the writer
exhibited at the first annual convention of the association of Michigan Engineers
and Surveyors at Lansing. The matter was further studied by Mr. Berger. The
screen was much improved, and the mechanical construction of it brought to
the standard of the work done by this firm. Mr. Berger soon conceived the
idea of making the screen of ground white glass in a brass frame, as shown in
figs. 1 and 2,*so one might observe the position of the images directly upon it,
and thus secure not only the comfort of an easy position in observing, but the
consequent accompanying accuracy. The arm of attachment was perfected from
time to time. The screen of ground glass is mounted upon an arm that admits of
all adjustments of position, and is so attached to the side of the telescope tube that
it can be turned up out of the way when not needed. The reflecting prism
can be screwed on to the eye-piece cap for observing at high altitudes. This also is
adjustable so as to look in any desired direction from the telescope tube. The
diagonal eye-piece also has its movable colored shade glass
as above stated. With
these attachments observations on the sun at all altitudes may be made in two
ways. By looking directly at it through the simple colored glass for low altitudes,
or through the prism and its shade glass for high altitudes. The other way is to
receive on the screen the images of the cross-wires and the sun and make the image
of the cross-wires just quarter the image of the sun by means of the slow motion
screws to the circles of the instrument. For this method the colored shade glasses
are not to be used. With this complete outfit one may work whichever way seems
bwst.
* For Figures 1, 2, 3, 4, 5, 6 and 7 see page 175.