They weren't meant to be seen. They were installed in series in a circuit to provide a "shock absorber" for when a station went off-hook during a ringing cycle so that a short wasn't placed on the central ringing generator. The lamp would glow, absorbing the ring voltage during the ring cycle. It is difficult to explain, but they provided a major improvement in ring voltage issues, like having it backfeed into DC circuits causing relay chatter.

Ed Vaughn, MBSWWYPBX