Why do we say 'her eyes had great clarity'?
Anatomically and from embryogenesis the eyes are an extension of the brain. The retina is a 6 layer neural network. For people in the mathematical sciences visual cognition has been coopted via notation and spacial constructs into a more general cognitive ability. Those with innate spacial intelligence often flourish in that realm. Further the lens, that which is clear or cloudy, has an extremely interesting vasculture; edge capillaries feed a central diffusion network (to keep the lens free of stray proteins and red corpuscles). Random deposits in the lens are likely to be correlated with deposits in the rest of the brain that have crossed the blood brain barrier (this is distributed physical-chemical barrier that gives neurons their own purified, haemless nutriment supply).
What can we say of people who wear glasses? Of people who score well on IQ tests, a disproportionate number have glasses (but not the other way around). One of the most powerful predictors of myopia is is (overly) rapid growth during embryogenesis of the visual system. When non-grain societies with no history of myopia are introduced to grain diets myopia sores. Typical polynesian rates are now 30-50%. This seems to be caused by IGF-1 (insulin related growth factor) bursts following ingestion of high GI grain products during pregnancy. Different parts of the lens and supporting tissue respond to these bursts at different rates, resulting in selective overdevelopment and global distortion. But IGF-1 has receptors all over the body and nervous system; it seems probable the unusual lens growth is reflected in the rest of the visual system. 3.7Kg is the average birthweight of an Australian infant. 4.2Kg brings an infant 3x more likey to develop myopia. But what hidden strengths and ailments does growth induced myopia also bring?
There are some related phrases with an obvious physiological basis to their etymology. e.g "flashing eyes", "wide eyed curiosity" and "focussed".