Original mirror for William Herschel's forty-foot telescope, 1785.

1785 in Slough
William Herschel

The first 48-inch speculum mirror cast by Sir William Herschel in 1785 for his 'Forty-Foot' telescope at Slough, England.

The astronomer William Herschel made this 48-inch concave metal mirror for his '40-Foot Telescope' at Slough, England. Cast in 1785, the mirror is made of speculum, a bronze alloy with arsenic for a more lustrous finish. Weighing only half a ton, the mirror proved to be too thin to hold its shape properly once it had been ground and polished to its final form. A few years later, a heavier second mirror was cast which was used in preference to the original one. Completed in 1787, Herschel's great telescope soon became a wonder of the age. It quickly became a landmark of Slough visited by the rich and famous visiting the king at nearby Windsor.

On display

Science Museum: Making the Modern World Gallery

If you are visiting to see this object, please contact us in advance to make sure that it will be on display.


Object Number:
1932-567 Pt1
iron and speculum
metal mirrors
Revd Sir John Charles William Herschel

Cite this page


We encourage the use and reuse of our collection data.

Data in the title, made, maker and details fields are released under Creative Commons Zero

Descriptions and all other text content are licensed under a Creative Commons Attribution 4.0 licence

Using our data


Download catalogue entry as json

View manifest in IIIF viewer

Add to Animal Crossing Art Generator

Download manifest IIIF

Our records are constantly being enhanced and improved, but please note that we cannot guarantee the accuracy of any information shown on this website.