Accession No


Brief Description

reflecting telescope, 18"/4" Gregorian, by James Short, English, circa 1758


London; England


Short. James


astronomy; optical

Earliest Date

Jan. 1, 1758

Latest Date

Dec. 31, 1758

Inscription Date


metal (brass, speculum metal); ivory; glass


telescope length 715mm; barrel length 618 mm (2 ft); height 801mm; breadth of base 335mm; diameter 111mm

Special Collection



‘James Short London 144/958=18.’ (on eye end)

Description Notes

Reflecting telescope, 18"/4" Gregorian, by James Short, English, circa 1758.

Gregorian brass reflecting telescope. 18-inch focal length; 4-inch aperture. 610mm-long barrel with friction tight cover and brass finder. Screw in eye end with screw in eye piece. Speculum metal primary mirror (d: 96mm) held by 3 springs. Focus by long screw from eye end, moving speculum metal secondary mirror. Linear focussing scale divided on brass 0 - 3”, numbered by 1 subdivided to 0.05. Barrel mounted on open half circle; altazimuth mount with 2 slow motion screws with ivory handles, operating by rack and pinion. Pillar and folding tripod stand, scroll feet. Altitude arc divided on brass 70 - 0 - 90 numbered by 10 subdivided to 1˚, with bress vernier. Manuscript instructions, probably in Short’s hand.

Condition poor (very tarnished); complete


Boris Jardine; 'A James Short telescope'; Explore Whipple Collections online article; Whipple Museum of the History of Science; University of Cambridge; 2006:


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


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