STANDARD - Survey Instrument Antique Center!

101
The Berger Short Focus Lens Attachment.
A very valuable addition to the engineer's outfit is found in the short focus lens
attachment which has been brought out. The contrivance is simple, but, like many
simple devices, is very effective in overcoming a practical difficulty. Probably every
engineer has been annoyed by being obliged to sight a point a little too near for the
telescope to focus. Most transit telescopes will not focus on a point much nearer than
6 or 6 feet (levels not nearer than 7 or 8 feet) away from the instrument, while it is
frequently necessary to sight a point on the ground nearly under the transit, at a distance
which is usually less than that.
In mine surveying as well as inside of factory buildings, one frequently needs to
sight a point overhead or on the walls and very near the transit. Ordinarily the only
way out of the difficulty is to focus as nearly as possible and do the rest by a guess.
As a further instance, one often finds in leveling, that it will be necessary to take
a reading on a point very near the instrument, and has to resort to various means
(all of them inaccurate) of getting around the difficulty. The attachment mentioned
consists of a small aluminum tube containing a simple lens, which is attached in
front of the objective. The lens is so placed in the tube that it can be accurately
centered by means of 4 adjusting screws. The effect of this lens is of course to bring
rays to a focus nearer to the objective, and thus enable the observer to focus a nearer
object than would otherwise be possible. When the telescope will focus no nearer than
6 feet, the attached lens, marked 1, is ground so that it will focus objects 6 feet away
when the objective tube is drawn away in. This allows the entire motion of the focusing
slide for distances between 6 and 4 feet. For distances nearer than 4 feet a second
lens may take the place of the first and will focus up to about 2| feet. If the two are
used at once the distance is reduced to about two feet.
With this pair of lenses there is no distance between two feet and infinity at which
objects cannot be focused. The accuracy of work done with this attachment is in no
way affected by the centering of the attached lens itself, as this is capable of perfect
adjustment. The only way in which error can occur is through the imperfection of
the objective tube. If the cylindrical surface of the object-head of the telescope on
which the attachment is placed is not concentric with the optical axis of the telescope
this error will enter into the adjustment of the attached short focus lens. This
error, however, is never large on an instrument sent out by our firm. But even
admitting that there may be some error here, it must be rememembered that this lens
is never used for objects more than about 6 feet away ; consequently the resulting
error on the point is entirely negligible, and the convenience of the attachment: 'n
many cases is so great that it entirely outweighs any such consideration, since the
work done at this distance will be entirely consistent with the work done with the
instrument on the longer distances. The attachment fills a want that has long been
felt by engineers and is certainly a step in advance in the perfection of instruments of
precision.
To attach this device to their old instruments it will be necessary to send the instrument
to them, as every lens attachment must be specially fitted and centered. How-
ever, it can be supplied with any of their new instruments, either Transits or Levels,
made since 1899.
When attached to transits, No. 1 permits focusing objects to about 3 feet, No. 2
permits focusing objects to about 2^ feet : both permit focusing objects to about 2 feet
from center of instrument.
This is so important a feature that one trial will convince one that it is indispensable
to the outfit of an engineer The device is patented. The Messrs. Berger
are also prepared to attach it to their Wye and Dumpy level, lor focusing nearly aa
close as stated above for transits. For pi-ices see catalogue, page 203.