火か線せん

noun

firing line

Military term for the line from which weapons are fired, or the line of fire itself.

• 兵へい士したちは火か線せんに立たたされた。

  The soldiers were placed on the firing line.

noun

caustic curve

Physics term for the envelope of light rays reflected or refracted by a curved surface.

• 火か線せんは光こう学がくで見みられる曲きょく線せんの一いっ種しゅだ。

  A caustic curve is a type of curve seen in optics.