I'd been working with this for the past few weeks to Tuan's suggestion. It was brutally hard when I was doing it with him on the other end of Skype. I tried doing it with a prism and could barely get the double images to superimpose even when using a seven diopter prism.
That was about a week and a half ago. Since we'd spoke I'd got much much better at it.I noticed a lot how much better I am at it today.
The idea is that you diverge your eyes so that the left eye fovea is on the left side target and the right eye fovea is on the right side target. When this is happening, you should see three balls (say, three footballs if you're on that line), because you're technically seeing four balls (two per eye, and you have two eyes), but one part left eye pair of balls is on top of one part of the right eye pair of balls. So the middle ball is a combination of the two, and the other two are seen just once by each eye.
Sounds a little confusing, right? Well it is a little confusing, at least at first.
One thing Dr. Tran mentioned is that when I diverge my eyes and try to fuse the images that it's important to try and get the double images to come in clearly. Well, that's pretty difficult to do. But he said something that struck me. 'Try to think about what it takes to make each eye come in clearly and then try and do that for both eyes simultaneously'. Sort of like singing and playing guitar simultaneously. Incidentally I never could sing and play guitar at the same time. But when I thought about it, there's no reason I shouldn't be able to do this. People override their accommodation reflex all of the time, particularly when they resolve those random dot stereograms (Magic Eye). This is a direct confrontation with the accommodation reflex.
The fact that he said that just made me realize that I should be able to do it, and so I started trying, and I'd been doing that for the past one and a half weeks, and I'd made very significant progress in that small amount of time.
I don't need any prism to get the double images to superimpose (for the bottom football). As you move up it gets more challenging (requires more divergence), and I've been able to get decent superposition at times with the soccer ball. I remember early last week it was damn near impossible to get the left eye pair to come in clearly until around the nine minute mark (I only do 10 minutes--it's a fairly strenuous workout). But now it takes less and less time for me to get both sets of images to come in clear. That is to say it is challenging. And as it gets easier there's a simple way for me to add to the challenge. Move up to the next set. When football gets too easy, move to soccer ball. As that gets easier, move to baseball.
You're supposed to get depth, but I haven't yet had any. My suspicion is that when I'm able to get both coming in perfectly clearly, I will get depth. I've gotten a very subtle sensation that I might have seen something that might have been the beginning of depth. It would make sense because it's only when you're focusing on something with both eyes simultaneously that you can truly seee through both eyes simultaneously. I imagine it will be an 'oh shit' moment.