Reader Brenda Philips, writing in Lodi, Italy, not far from Milan, asked: How many people wear glasses? More importantly, why are glasses so common now?

For now, we think Brenda's investigation is completely innocent and has nothing to do with the silly human tortoise, whose image is visible from the logo in this column.

The simple answer is that about two-thirds of American adults wear glasses or contact lenses of some kind. According to our analysis of more than 110,000 responses to the National Health Interview Survey conducted by the Census Bureau on behalf of the National Center for Health Statistics, 62 percent of respondents said they had worn some form of corrective lenses in the last three years.

But we have bad news for Brenda: The answer isn't as clear-cut as it sounds.

For one thing, it may not include everyone who wears frames or sunglasses because they look great, or because they've jumped on the blue-light bandwagon, or for any other reason that has nothing to do with vision correction.

Moreover, even among seemingly similar groups, the glasses were unevenly distributed. The ubiquity of glasses in your world changes depending on whether you're with a young legal worker (25 to 39 years old) or a friend who works in agriculture or construction. That's because legal workers are more than twice as likely to wear glasses.

What the hell is going on here? If, as we assume, good vision is inherited, how does your occupation determine your need for vision correction?

As we looked around for answers, we hit a small snag: the entire field of ophthalmology. The eye, it turns out, is extremely complicated, people have spent a lifetime studying it, and some jaded columnist and his database won't solve the problem once and for all in just a week.