This artifice of a double melting is often had recourse to, and especially in casting the alloys for the specula of telescopes.
I know of a gentleman who made a hundred of these specula with his own hands.
In the construction of reflecting telescopes, concave mirrors, or specula, are combined with a double convex lens.
For they had not only to design and make the specula, but also the mountings of the mirrors as well.
His writings on light, Perspectiva and specula mathematica, are included in his Opus majus.
The specula are made of speculum metal, which is a composition of certain proportions of copper and tin.
He studied science when he could, and hired a telescope, until he learnt to make his own specula and telescopes.
I do not know whether before that the Abb Rochon had thought of using the metal Platina for the specula telescope.
It also admits of as perfect a polish as the metal hitherto used for the specula of telescopes.
The specula Melitensis Encyclica gives an account of a kind of calculating machine of his invention.
speculum spec·u·lum (spěk'yə-ləm)
n. pl. spec·u·lums or spec·u·la (-lə)
A mirror or polished metal plate that is used as a reflector in optical instruments.
An instrument that is used to dilate the opening of a body cavity for medical examination.