Dent Meridian Instrument - Dipleidscope
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 Bottom w/3x5 card on face |
Background
In 1843 Bloxam patented a Meridian Instrument as Great Britian patent No. 9793. Bloxam brought his design to Dent who was an instrument maker and as a team they put the design into production. This instrument is very similar to Figure 12 through 19 in the patent.
Description
The key to operation are two
reflections of a star or the Sun. One reflection comes from the
clear glass on the front of the instrument. The second reflection
comes from the 2 mirrors that are at right angles behind the cover
glass. One reflection moves twice as fast as the other, and only
when the sun is on the meridian do the two images coincide.
The manual mentiones that when working with the Sun three measurements should be made. When the two images first touch each other, when the two images are congruent and when the two images are again just touching. By making three observations some error correction can be done and also this method allows for missing one of the observations maybe due to clouds.
The manual mentiones that when working with the Sun three measurements should be made. When the two images first touch each other, when the two images are congruent and when the two images are again just touching. By making three observations some error correction can be done and also this method allows for missing one of the observations maybe due to clouds.
The instrument is made from a Brass casting and is a little less than 2.5" high. There are a couple of notches on the bottom a little more than 0.2" wide and aboutt 1.25" C-C as well as a tapped hole that loosley accepts a 10-24 screw but is probably some British Standard thread. The notches and tapped hole would allow the Dipleidscope to be fixed in rotation.
The cap is engraved with the following:
E. I. Dent's
Patent
Meridian Instrument,
82, Strand,
& 33, Cockspur St,,
London
The two mirrors are rear surfaced, not front surface types. They are held in place by a set screw that presses on a metal plate that has raised lips at the ends so the mirror is clamped at it's ends. The angle between the mirrors is set by how the support brackets are machined and is not adjustable.
Operation
There is an image that is produced by light reflecting off of both mirrors, this image stays still as the instrument is rotated about the comon axis of the mirrors. To find this image you need to move your eye relative to the instrument. The other image is formed from a single reflection off of the cover glass, this image rotates as the instrument rotates.Back to Brooke's Products for Sale, Navigation, Sundials, Time & Frequency, Military Information, Home page
It is difficult to look at the Sun's image directly, both because it's bright but also because someone has polished the instrument to show off the Brass, but the origional instrument was painted flat black.
It can also be used as a reflecting ceiling dial, but to do so requires some type of mounting.
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