At night (or in poor visibility in daytime) an oil lamp displayed a white light when the signal was at 'all clear'; when the signal arm moved to the horizontal “danger” position it brought a red lens in front of the light, so that a red light was then displayed to oncoming trains. In poor visibility, GNR regulations called for detonators to be laid if possible on the track at the home signal when this was set to danger; at stations where platelayers were available, this should also be done at the distant signals.