Shaping, polishing and figuring of specula are accomplished by methods and tools very similar to those employed in the construction of lenses.
In 1778 he had selected the chef-d'oeuvre of some 400 specula which he made for the celebrated instrument of 7-ft.
The first constructor of reflecting telescopes on a large scale, William Herschel, never published anything about his methods of casting and polishing specula, and he does not appear to have been very successful beyond specula of 18 in.
Indeed, in its practical form the principle of the instrument has remained unchanged from the time of the Dollonds to the present day; and the history of its development may be summed up as consisting not in new optical discoveries but in utilizing new appliances for figuring and polishing, improved material for specula and lenses, more refined means of testing, and more perfect and convenient methods of mounting.
In Short's first telescopes the specula were of glass, as suggested by Gregory, but he afterwards used metallic specula only, and succeeded in giving to them true parabolic and elliptic figures.
The so-called first edition of the Speculum Majus, including the Speculum Morale, ascribed to Johann Mentelin and long celebrated as the earliest work printed at Strassburg, has lately been challenged as being only an earlier edition of Vincent's three genuine Specula (c. 1468-70), with which has been bound up the Speculum Morale first printed by Mentelin (c. 1 473-7 6).
The process of casting and annealing, in the case of the specula of the great Melbourne telescope, was admirably described by Dr Robinson in Phil.
The Specula Melitensis Encyclica (1638) gives an account of a kind of calculating machine of his invention.
Chiefly owing to the brittleness of this material, Lord Rosse's first larger specula were composed of a number of thin plates of speculum metal (sixteen for a 3-foot mirror) soldered on the back of a strong but light framework made of a peculiar kind of brass (2.75 of copper to 1 of zinc), which has the same expansion as his speculum metal.
Those silver-on-glass specula are now the rivals of the achromatic telescope, and it is not probable that many telescopes with metal specula will be made in the future.