L's dark eyes widened even before she finished speaking the name of the second substance; he didn't need to be told what it would do when it came in contact with fragile corneas.

L shook his head more rapidly as she approached, then touched Tony, brushing his hair aside in a way that could almost be considered tender in a different situation. If Tony died, he wouldn't need his eyes... but if Tony died, then L had nothing left to lose. Clearly, she was trying to capitalize on Tony's final moments and the amount of comfort he had access to. It wouldn't be like Moises' swift and abrupt end, it would be drawn-out and terrible and haunting.

You can't...

Except she could, and would. Tony's eyes burning out of their sockets wouldn't kill him yet, and until he breathed his last, he'd be maimed before the perfect eyes of someone who could have stopped it.

His mouth opened, but only to draw in a sharp breath his chest didn't want to let go of. To exhale would be to scream, perhaps, or would be if L would permit himself a single vocalization. To plead would be fruitless, anyway; her cold eyes said as much, succinctly and with chilling finality.