This arrangement consists in a cylindrical metal or horn mounted lens
two to four centimeters long, and magnifying two or three times, and
two or three centimeters in diameter, whose side is provided with a
contrivance for holding after it has been pushed into place a copying
needle, a protractor, etc.
While hitherto the architect in using millimeter paper must hold
separately in his hands a magnifying glass and needle, while the
engraver holds the engraving tool inclined in one hand and the
magnifying glass in the other, or must work under a large lens
standing on three feet, it is now possible by a firm connection
between the lens and needle or other instrument to draw directly with
one hand and under the lens. In the accompanying cut one of these
lenses is shown in section, A, in which the glass is set obliquely, in
whose focus the needle, _a_, is held and the field of view is
enlarged. A longer description is unnecessary, as the illustration
gives the best explanation. It need only be remarked that the stud,
_s_, projecting a little near the glass, is for the purpose of
preventing the instrument from leaving the position coinciding with
the plane of the drawing. For architects and engineers is provided a
small compass, _b_, of about 2 cm. diameter, for laying off parallel
widths, for making smaller scales and the like. In these cases it is
substituted for the needle. In like manner for calculating cross
profiles by graphical methods, for reading parallel divisions, for
estimating areas, or revising maps, a finely divided prismatic ivory
rule, _c_, can be placed under the glass, B, and will do good service.
In this case the plane of the lens must be perpendicular to the axis
of the tube.
[Illustration: IMPROVED DRAWING INSTRUMENT.]
For draughtsmen a parallel drawing pen, something like _b_, is used,
which gives several lines at once, perfectly parallel and close
together; or a drawing pen with which the smallest signatures, such as
boundary stones and figures, can be made neatly and exactly, which is
secured like the needle, _a_, and for which the cylinder serves also
as pen holder, offers a great advance.
Thus a whole series of instruments can be used with the lens. For
instance, a naturalist can use with it a knife or other instrument. To
avoid injury from the instruments, one should, in laying down the
cylinder, place it on its side. It is also recommended that on the
outer tube of the frame, which is appropriately lacquered of black
color, white arrows should be placed in the direction of the points of
the instrument, so that the eyes shall be protected from injury in
handling the instrument, as by the points being stuck into the pupil,
owing to lifting the instrument in an inverted position.--_Zeitschrift
fur Instrumentenkunde._
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