Yo go:

Clear two part epoxy has worked for me on multiple cracked 240 lenses, have used it successfully as more of a filler than as actual bonding agent.

Silicone-type stuff is water-proof and a great sealer but in my exp. doesn't adhere well to the hard-type plastic on a lens. It sticks well to softer plastics I find.

Epoxy seems to "attack" the plastic a bit as it were like Chris suggests. More of a "weld" than with silicone?

The clear two part epoxy is good stuff and works in apps. where things aren't flexed. Very strong, worked really well a while ago to weld a plasitc handle and tip onto fiberglass XC ski pole. No flex in the ends of the pole. Repair went > 5 yrs.

Am to the point where I must buy new lenses too--what's your source?

Frank in OH