Lens refractive index is higher, the thinner lenses, the greater the density, hardness and better, on the contrary, the lower the refractive index, the thicker the lens, the smaller the density, hardness is also poor, general glass of high hardness, Ita quod refractivae index est fere ad circiter 1.7, et resina film duritia est pauperer, in refractive index est relative humilis, resin pars in foro ad praesens est maxime commune refractive index in foro, paulo magis commune refractive de 1.499, leviter melius est ad plus-leviter versio Quod habet refractivam index circiter 1.56 et quoque usus est maxime.