Tabletop Telescope

Tabletop Telescope

RDS_treasures_447_Alment_Telescope.jpg

Although there are many variations between telescopes, they can be divided into two distinct categories − refracting and reflecting. Refracting telescopes use glass lenses to focus the incoming image. Reflecting telescopes use mirrors as the main element (though they also use lenses to focus images).

This telescope made by John Alment is a reflecting telescope, with a figured (i.e. ground to give a suitable concave surface) and polished ‘primary’ metal mirror of diameter 64mm at the base of the main tube. The tube is pointed at the image to be viewed, and the primary mirror directs the image to a much smaller ‘secondary’ mirror situated centrally at the front end of the main tube. This in turn reflects the image through a hole in the middle of the primary mirror and through a lens to give an image at the eyepiece, the arrangement of lens and eyepiece occupying a tube of much smaller diameter projecting from the bottom of the main tube. This arrangement is usually called a ‘Cassegrain’ reflecting telescope, named after a Frenchman who proposed the arrangement in 1672 . A dark red glass filter can be screwed over the eyepiece so that very bright images (e.g. the sun) can be viewed without damage to the eye. The telescope is focused by a knurled knob at the eyepiece end of the instrument, which moves the position of the secondary mirror at the top of the tube.

Looking down the main tube from the top: the circular metal ‘primary’ mirror, with a hole at its centre, is at the bottom of the tube, with the small ‘secondary’ mirror at the top. The position of the latter can be adjusted using the knurled knob at the bottom of the telescope.

The main tube is secured by two butterfly nuts to a pivoted support on top of a turned brass pillar, with folding cabriole tripod legs at the bottom. Though really not much more than a toy, which, though, would be particularly suitable for viewing the moon, the ensemble makes for a most attractive instrument.

It has a beautiful trade card of John Alment under the lid of its fitted oak case. The text on this trade card reads:

John Alment
Optician
At ye sign of ye Spectacles in
Marys Abbey Dublin
Makes Optical, Philosophical
& Mathematical Instruments
Viz Spectracles, Concave Glass,
Telescopes, Microscopes &c
Reading & Opera Glasses
Air Pumps, Electrical Machines,
Barometers, Thermometers, With
Variety of Drawing, Surveying
INSTRUMENTS

The trade card also has lovely instrument illustrations (anti-clockwise from top left): horizontal sun dial, universal ring dial, theodolite on tripod stand, spectacles, compound microscope, reflecting telescope (similar to the actual instrument in the case), four-draw refracting telescope; stick barometer (with two small unidentified instruments beside it), set of drawing instruments in shagreen case, and, in the centre, spectacles again.

It is not possible to know with certainty if such instruments were actually made by the named instrument maker (the telescope itself is not signed), whether they were assembled from parts made by others, or whether they were simply retailed. This type and size of telescope is not uncommon, and the instruments can be signed by a variety of different makers.

Mollan, C. (1997). Appendix. Most Recent Inventory of the RDS Scientific Instrument Collection.

RDS Treasures Collection

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RDS Library & Archives

Instrument

Ca.1770

Brass, glass, oak, paper

Tripod: 32 cm; Tube: 48 cm; Eyepiece: 8.5 cm