The real reason that Sophia was glancing at Jane like that is not revealed by this particular photo. Other photos of that event show very clearly that Jane’s top was not hiding her darker assets as she moved about.
Even that famous photo shows a "darker asset". I noticed that the first time I saw the pic years ago.
Funny how the culture has changed so much since my childhood. If Sophia and Jane were modern day celebs in the same scenario, Sophia would hardly notice those assets. If she did, she'd probably just smile.