Gauge glass, The light ray entering the steam or gas space strikes the groove face at an angle of 45o, being thereby refracted into the opposite groove face from which it is reflected back in direction of observation. The steam or gas space thereby appears silver-white. The liquid space behind the reflex glass appears black.

Gauge glass, The light ray entering the steam or gas space strikes the groove face at an angle of 45o, being thereby refracted into the opposite groove face from which it is reflected back in direction of observation. The steam or gas space thereby appears silver-white. The liquid space behind the reflex glass appears black.