TONIGHT begins the third week of what has proved the most successful comic opera produced in Boston for a good many years, Browne and Thompson's "The Sphinx," which is packing the commodious Tremont Theatre to the doors and creating a sensation seldom equalled in the regular season. The coolness of the Tremont is something unprecedented, the decorations unequalled, and the special features with which the management regales its patrons have all proved very successful. In the lobbies, between the acts, a beautiful Egyptian girl, Frommia, clad in the richest raiment of the East, dispenses from an Oriental booth Egyptian cigarettes to gentlemen; and in another part of the foyer, piano and vocal concerts are given at each intermission, making the broad halls a charming promenade.