( ( ( ( ( ( An ( iris diaphragm ) ) ( controls ( ( the amount ) ( of ( light ( entering ( the eye ) ) ) ) ) ) ) , ) and ) ( ( the pupil ) ( is ( a ( horizontal slit ) ) ) ) ) . )
(ROOT (S (S (NP (DT An) (NNS iris) (NN diaphragm)) (VP (VBZ controls) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NP (NN light)) (VP (VBG entering) (NP (DT the) (NN eye)))))))) (, ,) (CC and) (S (NP (DT the) (NN pupil)) (VP (VBZ is) (NP (DT a) (JJ horizontal) (NN slit)))) (. .)))
An iris diaphragm controls the amount of light entering the eye, and the pupil is a horizontal slit.
(ROOT (S (NP (NP (DT The) (NN pupil)) (PP (IN of) (NP (DT the) (NN eye)))) (VP (VBZ allows) (ADJP (JJ light) (S (VP (TO to) (VP (VB enter)))))) (. .)))
The pupil of the eye allows light to enter.