Accompanying this spacial quality is the sense that one is unable to clearly see the “center” of the jhana (the donut hole). This makes sense if we consider the middle to be “out of focus." As Daniel mentions, the middle is indicative of your whole body, which means you may also feel like you're transparent or vanishing from reality. This may sound frightening, but it is actually this dissolution which creates the 3rd jhana's pleasurable body relaxation, often described as “body buzz” or “body coolness.”
Thanks Trent I like the bit about the body being transparent, and I know what you mean. I had a period recently where I was sort of mistaking, the 2nd Jhana for the third and the possible the 3rd for the 4th. Any way that meant I was finding it very hard to access the fourth, because I wasn't sure what the third was, To me, one of the main distinctions between the third and the fourth is body bliss/pleasure and equanimity. In the fourth the bliss changes to equanimity and there is a unification of the mind.
i was hanging out in the third with that bodily bliss and spaciousness of attention and trying to cultivate rapture and then getting pulled back into the second as the focus on joy in the mind narrowed my field of attention, to me and away from the space and sounds around me. It's funny how in the third vipassana stage, things can get so intense, but be quite far-out and spacey and blissfully buzz in the 3rd jhana. I find it a lot easier to move between A&P and the 2nd jhana, but difficult making the shift with the third. To get the third quite good, I really need to go up through the jhana's more exclusively.