Silvered glass mirrors were a vast improvement over speculum metal and made reflectors a practical instrument.

He started building his own reflecting telescopes and would spend up to 16 hours a day grinding and polishing the speculum metal primary mirrors.

He chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror.

It was difficult to grind the speculum metal to a regular curvature.

For nearly 200 years speculum metal was the only mirror substance that could perform this task.

Rapidly cooling night time air would cause stresses in large speculum metal mirrors, distorting their shape and causing them to produce poor images.

Glass was also more thermally stable than speculum metal, allowing it to hold its shape better through temperature changes.

But speculum metal is a material of a very intractable description.

When completed, the disc of speculum metal was about six feet across and four inches thick.

The poor optical quality of the early polished speculum metal mirrors meant that telescopic sights were not practical.