Thanks Miss Nancy and Kawecki.

The shader I posted on the previous page is in fact just BB's thick glass shader, as far as I understand it... or a variant of it.

I've now used two color_math nodes to add together three HSV nodes, setup as RGB maximums of Red, Green and Blue. Each HSV node then has a separate refract node attached to its color channel.

For each refract node I've set a slightly higher IOR than the 1.53. of Crown Glass, and varied that by a fraction... e.g.

Red channel IOR=1.87, Green channel IOR=1.9 and Blue channel IOR=1.93.

This creates an approximation / representation of that chromatic aberration effect that EnglishBob explained. The extent of variation I'm just tweaking by eye, based on the scale of my render and how visible I want the effect to be.

The extent to which those Refract node IORs are set above the fresnel_blend node's IOR value, determines, in part, the extent of the magnification.

I also created a morph to vary how spherical the magnifying glass' lens is.

It seems to be working. Things close up behind the magnifying glass lens are magnified.

Place the glass at arms length from the camera with a more distant object behind it, and the glass produces a compressed and flipped view; a "real image", I understand the real world effect is called.