From line to 500V small signal diode , then bar or cathode to 47k to 1uF 250V cap non polar to neutral.
. Then from cap, mercury switch to 10k to gate of triac , then triac output1 to neutral, then output2 to light then to momentary line-rated off switch to line input.
Input diode RCR cct. creates high V DC to create constant gate On current at 1.2Vdc to gate when ON but half line voltage DC across mercury when off. Switch voltage rating must be given as high V or changes required with bigger cap, extra resistor divider across cap to < mercury switch rating.