( ( Consider ( ( ( ( first ( a ( converging lens ) ) ) , ) ( the ( ( most common ) type ) ) ) ( being ( a ( double ( convex lens ) ) ) ) ) ) . )
(ROOT (S (VP (VB Consider) (S (NP (NP (RB first) (DT a) (JJ converging) (NN lens)) (, ,) (NP (DT the) (ADJP (RBS most) (JJ common)) (NN type))) (VP (VBG being) (NP (DT a) (JJ double) (NN convex) (NN lens))))) (. .)))
Consider first a converging lens, the most common type being a double convex lens.
(ROOT (S (NP (DT The) (ADJP (RBS most) (JJ common)) (JJ two-lens) (NN telescope)) (, ,) (PP (IN like) (NP (DT the) (JJ simple) (NN microscope))) (, ,) (VP (VBZ uses) (NP (NN convex) (NNS lenses))) (. .)))