Huygenian eyepiece question

I realize most Astromarters are more concerned about modern eyepieces, but I still have a problem understanding why the primary image of the telescope objective falls between the lenses in a Huygenian eyepice. I am sure the answer is very simple but I cannot see the cause for it, when comparing diagrams of common older eyepieces. Is it the lens spacing? The relative focal lengths of the 2 lenses? or what? Thanks for humoring me.

Cheers,
Dave K.