Yeah, you’re right, I probably messed up the sliders… 
(But mind - the only realistic value for opacity is 1.0. Even a bubble should refract edges afair, since it’s a curved surface. Transparency kinda breaks the effect of realistic surface.)
So yeah, it all got me intrigued, and I tried to recreate this image with every - @seanwasere, @gonnavis, and Fresnel - approach, and in the end this glass ball is as far as I managed to get without a custom shader:
(1) @seanwasere approach, ie. 2 spheres + 2 env is the easiest and looks quite realistic enough. In a super-simplified version it technically needs just 3-4 params for materials (just don’t use opacity, that’s what envMaps and cubemap cameras are for. 
)
(2) As little as it makes sense I tried the experimental SSR branch, since I have it locally - but because it’s post-processing, reflections affect the rendered lights and it just looked a bit awkward. 
(the white light-dots on glass surface disappeared etc.)
(3) Fresnel shader looks by far the best - but first, it works only with cube map textures, second, it doesn’t seem to be affected by scene lights which kills the purpose. 
But it’d still be best to do a custom shader, since none of these could do the focused light effect on the other side of the ball (at least dynamically, and it could be easily calculated from sun position and IOR) :
