It is not uncommon to see this drink stipulated with equal parts gin, Chartreuse and sweet vermouth, as called for in Harry Johnson's Bartender’s Manual. In fact that's the norm. Equal parts works, but it makes for a drink strong enough to drop a rhinoceros with Chartreuse overly dominant (and I love Chartreuse).
The level of booze in this drink makes dilution key, so don’t hesitate to up the amount of water if using huge chunks of ice fresh out the freezer.
Recipe adapted from Harry Johnson's 1900 Bartender’s Manual which calls for “1/3 wine glass Chartreuse (green); 1/3 wine glass vermouth (Italian); 1/3 wine glass of Plymouth gin; 1 dash orange bitters”.
The Bijou first appeared in C. F. Lawor's 1895 book The Mixicologist as equal parts gin, Grand Marnier and sweet vermouth.