a biconcave lens

We were mostly concerned with the reflection and refraction of shocks in the unacoustic domain; we found that entirely new forms of these processes, connected with extraordinary pressures and production of vortex sheets, exist there.

Bid me go snatch a daring Thunderbolt, Or twi-fork'd lightning from the hand of Jove: […]

The axles of the Talgo section are U-shaped and the ends form stub-axles on which the wheels revolve freely and independently.