I'll be at the AGM next Sunday, and I'll have something for the raffle too.

The example of lenses below show a basic idea of how to do this.
I used a white undercoat, followed up with a transparent blue glaze. The lenses on the hooded chap shows a light coating of the blue, whereas the eye underneath the mask is a heavy coating.
Name:  OI000024.JPG
Views: 402
Size:  146.8 KB
Name:  OI000023.JPG
Views: 386
Size:  123.9 KB
Name:  OI000022.JPG
Views: 385
Size:  107.2 KB

I've also got some Army Painter Gloss Varnish (gives a subtle glass effect in the light), which I use on glass bottles/vials.

The below video shows an example of how to do lenses, you might find other videos more helpful, but this is an example of the above.
https://youtu.be/vLsteSBHEcM?t=895