If you came here looking for some deep literary stuff, sorry! This one is about literal truth (aka science).
Eyes, apparently, hold some dark secrets which may, some day soon, reveal themselves routinely to a skilled laser gazer. And, these secrets may be about some of the terrible things that could hit you well in the future: diabetes, hypertension (high blood pressure), heart disease, stroke. Piqued? Read this story.
And, another story claims that the eyes may also hold other gloomy secrets about a lurking Alzheimer's, long before it does permanent damage to the brain.