The magnifying glass was a glass that produced a magnified image of an object. Sun rays sent through a magnifying glass could be used to burn things. It was known to humans, Time Lords and Apalapucians. It was usually carried in a handheld tool.
The Doctor Edit
It was carried by the Doctor in several incarnations: first, (PROSE: A Big Hand for the Doctor) second, (TV: The Highlanders et al.) third, (TV: The Sea Devils et al.) fourth, (TV: Genesis of the Daleks et al.) fifth, (TV: Four to Doomsday) tenth, (TV: Partners in Crime et al.) and eleventh. (TV: The Crimson Horror)
Magnifying glass could amplify the heat from sun rays.
In one occasion, Donna Noble used the Doctor's glass this way against a Vespiform which then went out the window. When it appeared again later, the sight of Donna holding the magnifying glass, despite not being near sunlight, was enough to scare it off. (TV: The Unicorn and the Wasp)