( ( The ( diverging lens ) ) ( ( ( should again ) ( be ( a ( x20 ( objective ( or ( -8 ( mm ( double-concave lens ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (VBG diverging) (NN lens)) (VP (MD should) (ADVP (RB again)) (VP (VB be) (NP (DT a) (JJ x20) (NN objective) (CC or) (CD -8) (NN mm) (NN double-concave) (NN lens)))) (. .)))
The diverging lens should again be a x20 objective or -8mm double-concave lens.
(ROOT (S (NP (DT A) (JJ concave) (NN lens)) (VP (VBZ is) (NP (NP (JJR thicker)) (PP (IN at) (NP (DT the) (NNS edges)))) (SBAR (IN than) (S (NP (PRP it)) (VP (VBZ is) (PP (IN in) (NP (DT the) (NN middle))))))) (. .)))
A concave lens is thicker at the edges than it is in the middle.