I used put my finger halfway to the object, look at my finger until the crossed images in the background converge, then remove my finger and let my eyes stay in the same spot.
> until the crossed images in the background converge
that's the issue! like an OP i never was able to "converge" the images. for me it is phisically impossible: they are too far apart and never come closer. maybe because i have an astigmatism.
That's exactly it. You have to bring the "copy" of the left side to overlap with the "original" right side. Then move forward/backward until the overlapped 2 come into focus. The shimmering should stick out, was so shocking when it finally clicked for me.
So, how exactly is that supposed to work?
I can manage some de-focusing which makes me see 4 images rather than two. Is that part of it? Otherwise, I don't get it.