Accession No

1132

Brief Description

reflecting telescope, Gregorian, made by John Cuff, 1740 (c)

Origin

England; London; Fleet Street

Maker

Cuff, John

Class

astronomy; optical

Earliest Date

1740

Latest Date

1740

Inscription Date

Material

metal (brass); glass; leather

Dimensions

l (barrel) 635 mm; d (objective) 99 mm box length 820mm; breadth 261mm; height 190mm

Special Collection

Provenance

Purchased from R.S. Clay in 04/1952.

Inscription

JOHN CUFF FLEET STREET
LONDON

Description Notes

Gregorian telescope. Brass, black leather covered barrel. Screw in cover. Screw in eye end with screw on eyepiece shade. Threaded end holds objective by 3 brass springs. Speculum metal objective and secondary mirror moved by long screw with handle at eyepiece. Barrel held by 2 wing nuts to altazimuth mount, friction tight. Pillar stand, folding tripod base. Brass finder telescope with sliding covers for object glass and eyepiece. Fitted wooden box, 2 printed labels pasted on inside lid: “A brief description of the Gregorian reflecting telescope, as made and sold by John Cuff....” and a large trade label as at “Scientific trade cards” (Science Museum) no 101 (illustration 21)

Condition: Good

References

Events

Description
Isaac Newton built the first reflecting telescope in 1669. Unlike a refracting telescope a reflecting telescope uses concave mirrors to focus light. The great advantage of using mirrors is that all colours of light are reflected equally. This was an improvement on the refracting telescope which used lenses to focus light. Here the light refracted at different angles to create a distorting fringe of colour around the image being observed.

At first high quality reflecting telescopes were difficult to make as the mirrors available to use degraded quickly and were unable to transmit enough light. But by the eighteenth century methods of making mirrors had improved and instrument makers such as James Short had created hundreds of reflecting telescopes for sale.

There are three types of reflecting telescope: the Gregorian, the Cassegrain and the Newtonian. Both the Gregorian and Cassegrain telescopes are pointed directly at the object being viewed. But the Newtonian has a secondary mirror set at 45° to the objective so the observer stands at right angles to the telescope. Today modern telescopes (with apertures greater than one metre) are reflecting telescopes. Various innovations make them more powerful and accurate. Computer controlled deformable mirrors can correct against changes due to atmospheric conditions or faults in the material.

Created by: Jenny Downes

FM:39916