A belated answer and a new puzzle

I don't have any lenses in my eyes. Some time back, somebody asked me what keeps my aqueous humor from mingling with my vitreous humor, since the lens usually does that. (These are two different kinds of fluid in the eye.) I wondered if they combined, or if they had different viscosities (like oil and water) and so naturally separated, or what. This morning I finally remembered to ask my ophthalmologist.

Her answer was that it's basically the latter; the aqueous humor is watery (and, by the way, exits through a duct that is often the source of glaucoma problems -- it gets plugged up, raising intra-ocular pressure), while the vitreous "has the consistency of egg whites". However, she said, the vitreous thins out as we age; parts break off and become floaters (yup, got some of those...), and it tends to liquify. So yes, in my case they'll eventually blend, and perhaps they are already. But, she said, this doesn't actually hurt anything; both exist primarily to supply nutrients to the eye and they'll keep doing that regardless. Apparently the different parts of the eye can eat each others' food, or something. (That's my conclusion, not something she said directly.)

She seemed pleased to be able to answer a question more advanced than "do I need new glasses", too. :-)

And now the new "puzzle": I wonder why the eye chart, the set of letters you read on every visit to measure your distance vision, is always the same. I understand why it was, back in the days of paper charts. But now it's all digital -- yet, on every test, it's the same set of letters on each line, in decreasing size as you go on. I have to work pretty hard to avoid memorizing this so it'll be a fair test, since I see my doctor a few times a year (not just once a year like many people). I understand why they want to have a certain mix of letters; I'm sure it's quite intentional that they use a G that can be mistaken for a C, or an F that can be mistaken for a P, and so on. But why not mix it up? Is it important that the F be on the 20/40 line and the G on the 20/50 line and so on? In the same order?

I didn't ask that question.