weaver v24 problem,help me

[walrus]

This scope is a weaver v24 with the fine 1/8 target dot, AO, and focus.
My problem is that @ 100 yds my crosshairs disappear if I look at the target for longer than a few seconds. If I look away for a second and back then it reappears but again disappears in a few seconds.
The focus was preset at zero and I somehow turned it before I had it mounted. After mounting I tried to readjust it by looking at the sky and back thru the scope but can't see much differance. The focus adjustment is not marked so I can not tell where the zero of the focus is. Any thoughts?

[walrus]

This is actually a T24 and not a V24. Do I have the focus jacked up? I tried to adjust it several times with the same result.

[CatShooter]

Your eyepiece is not properly set.

Go here and read:

http://www.snipercountry.com/Articles/Parallax.asp

.