Telescope with altazimuth mount made by Tito Gonnella and presented to the Third Congress of Italian Scientists in Florence in 1841. Its manufacture should be viewed in the context of the heated controversy then raging between the supporters of refracting telescopes (which used lenses to form the astronomical image) and reflecting telescopes (which used mirrors). The limiting factor of telescopes with mirrors was the low reflectivity of the reflecting surfaces. The aluminizing techniques commonly employed today had not yet been developed. The silvering technique used at the time gave the reflecting surfaces a high reflectivity, but the effect faded very rapidly owing to silver oxidation. With this telescope, Gonnella proposed replacing the second mirror—typical of the Newtonian design—with a total-reflection prism that enhanced the telescope's luminosity by about 10 percent. This late innovation solved the problem only for small telescopes. At the same Congress, Giovanni Battista Amici also unveiled a large astronomical objective of his construction that gave images far more luminous than those produced by reflecting telescopes, even of much wider diameter.