Dent Meridian Instrument - Dipleidoscope

Brooke Clarke, N6GCE



Dent Meridian Instrument - Dipleidscope
Dent Meridian Instrument - Dipleidscope
Dent Meridian Instrument - Dipleidscope
Dent Meridian Instrument - Dipleidscope
CD-ROM Image
CD-ROM Image
CD-ROM Image
CD-ROM Image
CD-ROM Label
CD-ROM Window
CD-ROM Bottom
Bottom w/3x5 card on face


Background
Description
Operation
Patent
Related
Reference
Links

Background

In 1843 Bloxam patented a Meridian Instrument as Great Britain patent No. 9793.  Bloxam brought his design to Edward John Dent (Wiki) 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.  Dent's instrument manufacturing business is now called Dent London.  They are the company the build the clockwork for Big Ben (Wiki).

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 a sixty degree angle 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 mentions 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 about 1.25" C-C as well as a tapped hole that loosely 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 common 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.

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 original instrument was painted flat black.

It can also be used as a reflecting ceiling dial, but to do so requires some type of mounting.

Patent

US patent class 33/269 is Geometrical Instruments/Straight Line Ray Type - Celestial - Time Computing.

Reference

A Description of the Dipleidoscope, Or Double-reflecting Meridian ..., Volume 7  By Edward John Dent

Solid Dipleidoscope Prisms C.V. Boys - instead of using two mirrors.
Royal Museums Greenwich - Dent instrument with magnetic compass and time scale

The Museum of the History of Science: Dipleidoscope, by E. J. Dent, London, c. 1870 -


Post on  the Sundial Yahoo Group by Mike Cowham:
"The request for patent information sent me immediately to the booklet 'A Description of the Dipleidoscope...' by Edward Dent1843, a copy of which was supplied with his dipleidoscopes. It does not give the patent number, but one picture of the device E. I. DENT'S PATENT MERIDIAN INSTRUMENT giving the address 82 Strand & 33 Cockspur St.
The Postscript makes interesting reading and I have included it below.  I made a few photo copies that were sold on the BSS Bookstall at Dunchurch last year. If any more are required, I will make them available for a donation to the BSS of 1.50 pounds each. Anne Somerville will be proud of me.
+ + + + + + +
POSTSCRIPT
The origin of the Dipleidoscope resulted from the following circumstances. The writer had long felt persuaded that the interests of Horology would be promoted if the public were more generally possessed of a cheap, simple, and Correct transit-instrument, requiring little or no scientific knowledge for its right use, and not readily Susceptible of injury or derangement. To this end he had devoted much time and thought; and, in 1840, he considered that he had succeeded in inventing an apparatus which, by means of shadows, would produce the desiredresult. This idea he communicated to J. M. Bloxam, Esq., who thereupon informed him that his own attention had been for some years devoted to the same object, and that he had contrived an optical arrangement, which, by the agency of a single and double reflection, determined the sun's passage over the meridian with great exactness. When the optical instrument, although complicate in its then form, was shown to the writer, he was immediately struck with the superiority of the contrivance over that which had suggested itself to him: his own method afforded three observations, but it was attended with the defects and inconvenience which result from the uncertainty of shadows.  Convinced that the reflecting planes would effectually accomplish the desired end, he entered into an arrangement with Mr. Bloxam to undertake their manufacture; and, after nearly two years' attention on the part of that gentleman, and at great labour and expense on the part of the pro poser, they are now respectfully presented to the public in the present simple, but most accurate form. The writer, to secure his property in the instrument, as well as to insure its future perfect manufacture, solicited the favour of Mr. Bloxam to take out a patent in his own name, at the expense of the manufacturer, and, for a certain consideration, to transfer all interest in the invention to him. This request was kindly acceded to, and accordingly the Dipleidoscope, as an article of commerce, bears on it the name of the maker and proprietor, E. J. DENT.
Regards,
Mike Cowham.

Related

Dent's Portable Registering Tell Tale - Night Watchman's recording clock (added 11 April, 2017)

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